diff --git a/README.md b/README.md
index 45654a9..f0ed132 100644
--- a/README.md
+++ b/README.md
@@ -5,9 +5,10 @@
 
 This library implements parsers for various project scheduling benchmark instances, including:
 - Resource-Constrained Project Scheduling Problem (RCPSP)
-- Multi-Mode Resource-Constrained Project Scheduling Problem (MMRCPSP)
-- Resource-Constrained Project Scheduling Problem with Minimal and Maximal Time Lags (RCPSP/max)
+- Multi-Mode RCPSP (MMRCPSP)
+- RCPSP with Minimal and Maximal Time Lags (RCPSP/max)
 - Resource-Constrained Multi Project Scheduling Problem (RCMPSP)
+- RCPSP with flexible project structure (RCPSP-PS) and RCPSP with alternative subgraphs (RCPSP-AS)
 
 `psplib` has no dependencies and can be installed in the usual way:
 
@@ -47,6 +48,11 @@ To parse a specific instance format, set the `instance_format` argument in `pars
 2. `patterson`: The **Patterson format**: used for RCPSP instances, mostly used by the [OR&S](https://www.projectmanagement.ugent.be/research/data) library. See [this](http://www.p2engine.com/p2reader/patterson_format) website for more details.
 3. `rcpsp_max`: The **RCPSP/max format** is used for RCPSP/max instances from [TU Clausthal](https://www.wiwi.tu-clausthal.de/en/ueber-uns/abteilungen/betriebswirtschaftslehre-insbesondere-produktion-und-logistik/research/research-areas/project-generator-progen/max-and-psp/max-library/single-mode-project-duration-problem-rcpsp/max).
 4. `mplib`: The **MPLIB format** is used for RCMPSP instances from the [MPLIB](https://www.projectmanagement.ugent.be/research/data) library.
+5. `rcpsp_ps`: The **RCPSP-PS format** is the format used by [Van der Beek et al. (2024)](https://www.sciencedirect.com/science/article/pii/S0377221724008269).
+Specifically, we included an extra line that defines for each task whether it is optional or not.
+6. `aslib`: The **ASLIB format** is the format used by RCPSP-AS instances from the ASLIB instance set at [OR&S project database](https://www.projectmanagement.ugent.be/research/data).
+ASLIB consist of three different parts (a, b, c). 
+To use this parser, you have to merge parts (a) and (b) into a single file - part (c) is not parsed.
 
 ## Instance databases
 
diff --git a/data/aslib0_0.rcp b/data/aslib0_0.rcp
new file mode 100644
index 0000000..d90416f
--- /dev/null
+++ b/data/aslib0_0.rcp
@@ -0,0 +1,251 @@
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diff --git a/data/rcpsp_ps.txt b/data/rcpsp_ps.txt
new file mode 100644
index 0000000..dede5c8
--- /dev/null
+++ b/data/rcpsp_ps.txt
@@ -0,0 +1,546 @@
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diff --git a/example.ipynb b/example.ipynb
index 224182f..2eec68c 100644
--- a/example.ipynb
+++ b/example.ipynb
@@ -19,7 +19,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 52,
+   "execution_count": 1,
    "id": "4fec4d5b-df0f-48c4-9e55-602203a89176",
    "metadata": {},
    "outputs": [],
@@ -45,7 +45,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 53,
+   "execution_count": 2,
    "id": "0530bec2-580c-4845-a1f7-8f94d69a5f40",
    "metadata": {},
    "outputs": [],
@@ -63,7 +63,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 54,
+   "execution_count": 3,
    "id": "bfbca94a-720a-4873-b1b7-3c0a52d2a5b0",
    "metadata": {},
    "outputs": [
@@ -73,7 +73,7 @@
        "(4, 32)"
       ]
      },
-     "execution_count": 54,
+     "execution_count": 3,
      "metadata": {},
      "output_type": "execute_result"
     }
@@ -92,7 +92,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 55,
+   "execution_count": 4,
    "id": "e2f6acd6-f529-4833-b479-e00550584820",
    "metadata": {},
    "outputs": [
@@ -105,7 +105,7 @@
        " Resource(capacity=12, renewable=True)]"
       ]
      },
-     "execution_count": 55,
+     "execution_count": 4,
      "metadata": {},
      "output_type": "execute_result"
     }
@@ -124,20 +124,20 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 56,
+   "execution_count": 5,
    "id": "8a1235ac-d44f-4d01-9081-dfb7778c2fc7",
    "metadata": {},
    "outputs": [
     {
      "data": {
       "text/plain": [
-       "[Activity(modes=[Mode(duration=0, demands=[0, 0, 0, 0])], successors=[1, 2, 3], delays=None, name=''),\n",
-       " Activity(modes=[Mode(duration=8, demands=[4, 0, 0, 0])], successors=[5, 10, 14], delays=None, name=''),\n",
-       " Activity(modes=[Mode(duration=4, demands=[10, 0, 0, 0])], successors=[6, 7, 12], delays=None, name=''),\n",
-       " Activity(modes=[Mode(duration=6, demands=[0, 0, 0, 3])], successors=[4, 8, 9], delays=None, name='')]"
+       "[Activity(modes=[Mode(duration=0, demands=[0, 0, 0, 0])], successors=[1, 2, 3], delays=None, optional=False, selection_groups=[], name=''),\n",
+       " Activity(modes=[Mode(duration=8, demands=[4, 0, 0, 0])], successors=[5, 10, 14], delays=None, optional=False, selection_groups=[], name=''),\n",
+       " Activity(modes=[Mode(duration=4, demands=[10, 0, 0, 0])], successors=[6, 7, 12], delays=None, optional=False, selection_groups=[], name=''),\n",
+       " Activity(modes=[Mode(duration=6, demands=[0, 0, 0, 3])], successors=[4, 8, 9], delays=None, optional=False, selection_groups=[], name='')]"
       ]
      },
-     "execution_count": 56,
+     "execution_count": 5,
      "metadata": {},
      "output_type": "execute_result"
     }
@@ -156,7 +156,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 57,
+   "execution_count": 6,
    "id": "33dec608-c15c-4b7b-894d-0a6f67d8dafc",
    "metadata": {},
    "outputs": [
@@ -166,7 +166,7 @@
        "[Mode(duration=0, demands=[0, 0, 0, 0])]"
       ]
      },
-     "execution_count": 57,
+     "execution_count": 6,
      "metadata": {},
      "output_type": "execute_result"
     }
@@ -194,7 +194,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 58,
+   "execution_count": 7,
    "id": "7c44dee1-a7d8-4c5f-9d5c-9dc119bb597d",
    "metadata": {},
    "outputs": [],
@@ -220,7 +220,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 60,
+   "execution_count": 8,
    "id": "2a0012ed-f019-4aaf-85b3-93241d380401",
    "metadata": {},
    "outputs": [],
@@ -238,20 +238,20 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 61,
+   "execution_count": 9,
    "id": "6cdb8e24-587f-4b4b-b836-d808a3ecbe3e",
    "metadata": {},
    "outputs": [
     {
      "data": {
       "text/plain": [
-       "[Activity(modes=[Mode(duration=0, demands=[0, 0, 0, 0, 0])], successors=[3, 2, 1, 8], delays=[0, 0, 0, 0], name=''),\n",
-       " Activity(modes=[Mode(duration=2, demands=[5, 7, 8, 4, 6])], successors=[10], delays=[2], name=''),\n",
-       " Activity(modes=[Mode(duration=9, demands=[10, 8, 0, 8, 10])], successors=[4, 11, 7], delays=[5, 9, 0], name=''),\n",
-       " Activity(modes=[Mode(duration=6, demands=[9, 9, 0, 4, 5])], successors=[9], delays=[3], name='')]"
+       "[Activity(modes=[Mode(duration=0, demands=[0, 0, 0, 0, 0])], successors=[3, 2, 1, 8], delays=[0, 0, 0, 0], optional=False, selection_groups=[], name=''),\n",
+       " Activity(modes=[Mode(duration=2, demands=[5, 7, 8, 4, 6])], successors=[10], delays=[2], optional=False, selection_groups=[], name=''),\n",
+       " Activity(modes=[Mode(duration=9, demands=[10, 8, 0, 8, 10])], successors=[4, 11, 7], delays=[5, 9, 0], optional=False, selection_groups=[], name=''),\n",
+       " Activity(modes=[Mode(duration=6, demands=[9, 9, 0, 4, 5])], successors=[9], delays=[3], optional=False, selection_groups=[], name='')]"
       ]
      },
-     "execution_count": 61,
+     "execution_count": 9,
      "metadata": {},
      "output_type": "execute_result"
     }
@@ -278,7 +278,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 63,
+   "execution_count": 10,
    "id": "3280aeae-0bcd-4faa-8649-6a8b64db4e5e",
    "metadata": {},
    "outputs": [],
@@ -288,7 +288,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 64,
+   "execution_count": 11,
    "id": "f3d65f88-1840-4665-abcb-3d1fd3ae46da",
    "metadata": {},
    "outputs": [
@@ -298,7 +298,7 @@
        "(4, 372, 6)"
       ]
      },
-     "execution_count": 64,
+     "execution_count": 11,
      "metadata": {},
      "output_type": "execute_result"
     }
@@ -317,7 +317,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 68,
+   "execution_count": 12,
    "id": "67e3f2ad-8aed-47fe-9a43-cbe7159a585d",
    "metadata": {},
    "outputs": [
@@ -328,7 +328,7 @@
        " Project(activities=[62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123], release_date=0)]"
       ]
      },
-     "execution_count": 68,
+     "execution_count": 12,
      "metadata": {},
      "output_type": "execute_result"
     }
@@ -336,6 +336,227 @@
    "source": [
     "instance.projects[:2]"
    ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "0d50356a-172d-4086-95e0-afd838ea7c56",
+   "metadata": {},
+   "source": [
+    "## RCPSP-PS and ASLIB format\n",
+    "In the RCPSP with flexible project structures (RCPSP-PS), there are choices about which activities to schedule.\n",
+    "\n",
+    "1. Activities can be optional - they don't all need to be scheduled\n",
+    "2. Activities each have a set of associated selection groups\n",
+    "3. If an activity is scheduled, then for each selection group, exactly one activity must also be scheduled"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "ba6e61af-bd49-4871-b932-a78e23d24e0b",
+   "metadata": {},
+   "source": [
+    "Our library support two common RCPSP-PS formats."
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "a915ad69-962b-43f4-bc99-002739bb7eb8",
+   "metadata": {},
+   "source": [
+    "### RCPSP-PS"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "68832a0e-eca0-407e-ad3b-dad79bc72cc4",
+   "metadata": {},
+   "source": [
+    "The \"RCPSP-PS\" format is the format used by [Van der Beek et al. (2024)](https://www.sciencedirect.com/science/article/pii/S0377221724008269)."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 13,
+   "id": "c7b74459-6107-4e88-a729-7f0cc7d7f76f",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "instance = parse(\"data/rcpsp_ps.txt\", instance_format=\"rcpsp_ps\")"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 14,
+   "id": "bebe5e60-c84a-45c4-a9ef-9d7f34eed561",
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "(4, 136)"
+      ]
+     },
+     "execution_count": 14,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "(instance.num_resources, instance.num_activities)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 15,
+   "id": "990c6936-cbf2-47c4-823d-35fbdc2c240f",
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "Activity(modes=[Mode(duration=0, demands=[0, 0, 0, 0])], successors=[1, 2], delays=None, optional=False, selection_groups=[[1, 2]], name='')"
+      ]
+     },
+     "execution_count": 15,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "activity = instance.activities[0]  # source\n",
+    "activity"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "529e7add-9c25-4a41-b44b-4fda01f2f5d0",
+   "metadata": {},
+   "source": [
+    "The source activity must always be present (`optional=False`).\n",
+    "It has one selection group consisting of activities 1 and 2.\n",
+    "This means that either activity 1 or activity 2 must be scheduled."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 16,
+   "id": "1cb5b8c1-a6f3-4297-a327-fb2b4f05a927",
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "Activity(modes=[Mode(duration=0, demands=[0, 0, 0, 0])], successors=[3, 4], delays=None, optional=True, selection_groups=[[3], [4]], name='')"
+      ]
+     },
+     "execution_count": 16,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "activity = instance.activities[1]\n",
+    "activity"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "a955cdd4-fb27-491f-8ebd-d26737c9c9d7",
+   "metadata": {},
+   "source": [
+    "This activity has two selection groups, one with activity 3, and the other with activity 4.\n",
+    "If this activity is scheduled, then both activity 3 and activity must be scheduled."
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "01d7ae4f-58c3-445b-899c-2919a0fd3a79",
+   "metadata": {},
+   "source": [
+    "### ASLIB\n",
+    "The \"ASLIB\" format is the format used for RCPSP with alternative subgraph (RCPSP-AS) instances at https://www.projectmanagement.ugent.be/research/project_scheduling/rcpspas.\n",
+    "\n",
+    "Like the RCPSP-PS, the RCPSP-AS also has optional tasks and selection groups, but they use slightly different concepts (branches and subgraphs). \n",
+    "\n",
+    "**Important**: RCPSP-AS instances from the ASLIB instance set contain multiple file parts (a, b, c).\n",
+    "Our library parses _merged part (a) and (b)_ files, meaning that you have to manually concatenate files first.\n",
+    "See the instance file for details."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 17,
+   "id": "e9a53c91-5102-4a7a-8768-24fa645370c2",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "instance = parse(\"data/aslib0_0.rcp\", instance_format=\"aslib\")"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 18,
+   "id": "26e640fe-bac2-41dc-8761-f645d144dc74",
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "(5, 122)"
+      ]
+     },
+     "execution_count": 18,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "(instance.num_resources, instance.num_activities)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 19,
+   "id": "21d245eb-c3b4-4717-9597-f1850e3f9689",
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "Activity(modes=[Mode(duration=0, demands=[0, 0, 0, 0, 0])], successors=[1, 13, 25, 37, 49], delays=None, optional=False, selection_groups=[[1, 13, 25, 37, 49]], name='')"
+      ]
+     },
+     "execution_count": 19,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "activity = instance.activities[0]  # source\n",
+    "activity"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 20,
+   "id": "889d69b0-b492-4423-9d9d-64bac9875141",
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "Activity(modes=[Mode(duration=0, demands=[0, 0, 0, 0, 0])], successors=[2, 3, 4, 5, 6, 7], delays=None, optional=True, selection_groups=[[2], [3], [4], [5], [6], [7]], name='')"
+      ]
+     },
+     "execution_count": 20,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "activity = instance.activities[1]\n",
+    "activity"
+   ]
   }
  ],
  "metadata": {
diff --git a/formats/README.md b/formats/README.md
new file mode 100644
index 0000000..2ea616b
--- /dev/null
+++ b/formats/README.md
@@ -0,0 +1,3 @@
+# Instance formats
+
+This folder contains instructions for each instance format.
diff --git a/formats/aslib.md b/formats/aslib.md
new file mode 100644
index 0000000..90ee97a
--- /dev/null
+++ b/formats/aslib.md
@@ -0,0 +1,5 @@
+# ASLIB
+
+ASLIB files contain two parts (A) and (B).
+See https://www.projectmanagement.ugent.be/research/project_scheduling/rcpspas for details.
+The `psplib` library expects a single file which includes the merged parts of (A) and (B).
diff --git a/formats/rcpsp_ps.md b/formats/rcpsp_ps.md
new file mode 100644
index 0000000..900ce8c
--- /dev/null
+++ b/formats/rcpsp_ps.md
@@ -0,0 +1,57 @@
+Note: taken from https://data.4tu.nl/articles/dataset/Instances_and_file_format_for_the_Resource_Constrained_Project_Scheduling_Problem_with_a_flexible_Project_Structure/21106768 
+
+----
+
+The input file format is as following (# of means number of)
+Counting starts at 0
+
+	[# of activities] [# of renewable resources] [#number of nonrewable resources]
+	[resource availabilities]
+
+	[duration] [resource requirements per resource type]
+	[# of selection-groups] [[# of or selection successors] [selection group successor1] [selection group successor2]...] (repeat)
+	[# of time precedence successors] [successor1] [successor2] ...
+
+
+
+
+To give a small example:
+
+	5 1 0
+	10
+	
+	0 0
+	2 2 1 2 1 3
+	3 1 2 3
+	
+	2 5
+	1 1 4
+	1 4
+	
+	2 6
+	1 1 4
+	1 4
+	
+	3 10
+	1 1 4
+	1 4
+	
+	4 0
+	0
+	0
+	
+This means:
+
+	5 1 0
+	10 6 8 2
+	1.05 1.06
+	
+In total 5 activities, with 1 renewable resource (with availability 10), and 0 nonrenewable resources
+
+	0 0
+	2 2 1 2 1 4
+	3 1 2 3
+	
+First line: Activity 0 with a duration of 0 and no resource requirements
+Second line: Two selection-groups. The first one has two successor nodes 
+Third line: Three time successors: Activities 1 2 and 3
diff --git a/psplib/ProjectInstance.py b/psplib/ProjectInstance.py
index 5e8abf3..d483298 100644
--- a/psplib/ProjectInstance.py
+++ b/psplib/ProjectInstance.py
@@ -1,4 +1,4 @@
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 from typing import Optional
 
 
@@ -52,6 +52,12 @@ class Activity:
         the length of this list must be equal to the length of `successors`.
         Delays are used for RCPSP/max instances, where the precedence
         relationship is defined as ``start(pred) + delay <= start(succ)``.
+    optional
+        Whether this activity is optional or not. Default ``False``.
+    selection_groups
+        The selection groups of this activity. If the current activity is
+        scheduled, then for each group, exactly one activity must be scheduled.
+        This is used for RCPSP-PS instances. Default is an empty list.
     name
         Optional name of the activity to identify this activity. This is
         helpful to map this activity back to the original problem instance.
@@ -60,6 +66,8 @@ class Activity:
     modes: list[Mode]
     successors: list[int]
     delays: Optional[list[int]] = None
+    optional: bool = False
+    selection_groups: list[list[int]] = field(default_factory=list)
     name: str = ""
 
     def __post_init__(self):
diff --git a/psplib/__init__.py b/psplib/__init__.py
index 4af003b..7193308 100644
--- a/psplib/__init__.py
+++ b/psplib/__init__.py
@@ -1,6 +1,8 @@
 from .parse import parse as parse
+from .parse_aslib import parse_aslib as parse_aslib
 from .parse_mplib import parse_mplib as parse_mplib
 from .parse_patterson import parse_patterson as parse_patterson
 from .parse_psplib import parse_psplib as parse_psplib
 from .parse_rcpsp_max import parse_rcpsp_max as parse_rcpsp_max
+from .parse_rcpsp_ps import parse_rcpsp_ps as parse_rcpsp_ps
 from .ProjectInstance import ProjectInstance as ProjectInstance
diff --git a/psplib/parse.py b/psplib/parse.py
index 9e072c1..dd1bb59 100644
--- a/psplib/parse.py
+++ b/psplib/parse.py
@@ -1,10 +1,12 @@
 from pathlib import Path
 from typing import Union
 
+from .parse_aslib import parse_aslib
 from .parse_mplib import parse_mplib
 from .parse_patterson import parse_patterson
 from .parse_psplib import parse_psplib
 from .parse_rcpsp_max import parse_rcpsp_max
+from .parse_rcpsp_ps import parse_rcpsp_ps
 from .ProjectInstance import ProjectInstance
 
 
@@ -35,5 +37,9 @@ def parse(
         return parse_rcpsp_max(loc)
     elif instance_format == "mplib":
         return parse_mplib(loc)
+    elif instance_format == "rcpsp_ps":
+        return parse_rcpsp_ps(loc)
+    elif instance_format == "aslib":
+        return parse_aslib(loc)
 
     raise ValueError(f"Unknown instance format: {instance_format}")
diff --git a/psplib/parse_aslib.py b/psplib/parse_aslib.py
new file mode 100644
index 0000000..73f0ada
--- /dev/null
+++ b/psplib/parse_aslib.py
@@ -0,0 +1,224 @@
+from collections import defaultdict, deque
+from dataclasses import dataclass
+from itertools import chain
+from pathlib import Path
+from typing import Union
+
+from .ProjectInstance import (
+    Activity,
+    Mode,
+    Project,
+    ProjectInstance,
+    Resource,
+)
+
+
+@dataclass
+class AlternativeSubgraph:
+    """
+    Represents a single alternative subgraph in an ASLIB instance.
+
+    Parameters
+    ----------
+    branches
+        A list of branches in the subgraph. Each branch is a list of activity
+        indices that are part of the branch.
+    """
+
+    branches: list[list[int]]
+
+
+def _parse_part_a(lines):
+    """
+    Part (a) of ASLIB instance is formatted as Patterson instance.
+    """
+    num_activities, num_resources = map(int, next(lines).split())
+
+    # Instances without resources do not have an availability line.
+    capacities = list(map(int, next(lines).split())) if num_resources else []
+    resources = [Resource(capacity=cap, renewable=True) for cap in capacities]
+
+    activities = []
+    for _ in range(num_activities):
+        values = map(int, next(lines).split())
+        duration = int(next(values))
+        demands = [int(next(values)) for _ in range(num_resources)]
+        num_successors = int(next(values))
+        successors = [int(next(values)) - 1 for _ in range(num_successors)]
+        activities.append(Activity([Mode(duration, demands)], successors))
+
+    return resources, activities
+
+
+def _parse_part_b(lines) -> list[AlternativeSubgraph]:
+    """
+    Part (b) of ASLIB instance results in alternative subgraphs.
+    """
+    pct_flex, pct_nested, pct_linked = map(float, next(lines).split())
+    num_subgraphs = int(next(lines))
+    total_branches = 1  # first branch is always the dummy branch
+    subgraphs = []
+
+    for _ in range(num_subgraphs):
+        num_branches, *branch_idcs = map(int, next(lines).split())
+        total_branches += num_branches
+        branch_idcs = [idx - 1 for idx in branch_idcs]
+        subgraphs.append(branch_idcs)
+
+    branches: list[list[int]] = [[] for _ in range(total_branches)]
+    for activity, line in enumerate(lines):
+        num_braches, *branch_idcs = map(int, line.split())
+        for idx in branch_idcs:
+            branches[idx - 1].append(activity)
+
+    # Return the alternative subgraphs, with the first subgraph containing the
+    # fixed activities (an activity belongs to node branch 0 if it is fixed).
+    result = [AlternativeSubgraph([branches[0]])]
+    result += [
+        AlternativeSubgraph([branches[idx] for idx in branch_idcs])
+        for branch_idcs in subgraphs
+    ]
+
+    return result
+
+
+def parse_aslib(loc: Union[str, Path]) -> ProjectInstance:
+    """
+    Parses an ASLIB-formatted instance from a file. This format is used for
+    RCPSP instances with alternative subgraphs.
+
+    Note
+    ----
+    This function parses files that combine both "a" and "b" part files from
+    the ASLIB instance. You have to manually create such instances first!
+
+    Parameters
+    ----------
+    loc
+        The location of the instance.
+
+    Returns
+    -------
+    ProjectInstance
+        The parsed project instance.
+    """
+    with open(loc, "r") as fh:
+        lines = iter(line.strip() for line in fh.readlines() if line.strip())
+
+    resources, activities = _parse_part_a(lines)
+    subgraphs = _parse_part_b(lines)
+
+    # With the already parsed activities and alternative subgraph data,
+    # we add optional and selections groups data to the activities.
+    activities = _make_optional_activities(activities, subgraphs)
+
+    project = Project(list(range(len(activities))))
+    return ProjectInstance(resources, activities, [project])
+
+
+class DiGraph:
+    """
+    Simple directed graph implementation to replace networkx.DiGraph.
+    """
+
+    def __init__(self):
+        self.adj: dict[int, list[int]] = defaultdict(list)
+        self.nodes = set()
+
+    def add_node(self, node: int):
+        self.nodes.add(node)
+
+    def add_edge(self, u: int, v: int):
+        self.adj[u].append(v)
+        self.nodes.add(u)
+        self.nodes.add(v)
+
+    def topological_sort(self) -> list[int]:
+        """
+        Returns a topological ordering of the graph's nodes.
+        """
+        in_degree = {node: 0 for node in self.nodes}
+        for u in self.adj:
+            for v in self.adj[u]:
+                in_degree[v] += 1
+
+        queue = deque(node for node in self.nodes if in_degree[node] == 0)
+        order = []
+        while queue:
+            node = queue.popleft()
+            order.append(node)
+            for neighbor in self.adj[node]:
+                in_degree[neighbor] -= 1
+                if in_degree[neighbor] == 0:
+                    queue.append(neighbor)
+
+        return order
+
+    @classmethod
+    def from_activities(cls, activities: list[Activity]):
+        G = cls()
+        for idx, activity in enumerate(activities):
+            G.add_node(idx)
+            for succ in activity.successors:
+                G.add_edge(idx, succ)
+
+        return G
+
+
+def _make_optional_activities(
+    activities: list[Activity],
+    subgraphs: list[AlternativeSubgraph],
+) -> list[Activity]:
+    """
+    Adds optional and selection group data to activities based on alternative
+    subgraph data. Because the activity graphs are directed acylcic, we can
+    transform subgraphs into selection groups.
+    """
+    G = DiGraph.from_activities(activities)
+    order = G.topological_sort()
+
+    is_fixed = subgraphs[0].branches[0]  # first subgraph contains fixed nodes
+    alternatives = subgraphs[1:]
+    all_branching = []  # idcs of branching activities
+    groups = defaultdict(list)
+
+    for subgraph in alternatives:
+        # The branching activities are the lowest-indexed activities in each
+        # branch. This works because we have a directed acyclic graph.
+        branching = [min(b, key=order.index) for b in subgraph.branches]
+        arcs = [(u, v) for u in G.adj for v in G.adj[u] if v in branching]
+
+        # The principal activity is the sole activity that goes to the
+        # branching activities.
+        nodes = {u for (u, _) in arcs}
+        assert len(nodes) == 1  # should be only 1 principal activity
+        principal = nodes.pop()
+
+        all_branching.extend(branching)
+        groups[principal].append(branching)
+
+    # For all remaining edges, we add another unit selection group if
+    # v is not a branching activity. Edges with v as a branching activity
+    # are already covered by the groups above.
+    for u in G.adj:
+        for v in G.adj[u]:
+            if v not in all_branching:
+                groups[u].append([v])
+
+    # Create new activities with optional and selection group data.
+    new = []
+    for idx, activity in enumerate(activities):
+        activity = Activity(
+            modes=activity.modes,
+            successors=activity.successors,
+            optional=idx not in is_fixed,
+            selection_groups=groups[idx],
+        )
+        new.append(activity)
+
+    for activity in new:
+        # Check: In RCPSP-AS, timing successors are also selection successors.
+        select_succ = list(chain(*activity.selection_groups))
+        assert sorted(select_succ) == sorted(activity.successors)
+
+    return new
diff --git a/psplib/parse_rcpsp_ps.py b/psplib/parse_rcpsp_ps.py
new file mode 100644
index 0000000..fd276c6
--- /dev/null
+++ b/psplib/parse_rcpsp_ps.py
@@ -0,0 +1,44 @@
+from pathlib import Path
+from typing import Union
+
+from .ProjectInstance import Activity, Mode, Project, ProjectInstance, Resource
+
+
+def parse_rcpsp_ps(instance_loc: Union[str, Path]) -> ProjectInstance:
+    """
+    Parses a RCPSP-PS formatted instance from Van der Beek et al. (2024).
+    """
+    with open(instance_loc, "r") as fh:
+        lines = iter(line.strip() for line in fh.readlines() if line.strip())
+
+    num_activities, num_renewable, _ = map(int, next(lines).split())
+    capacities = list(map(int, next(lines).split()))
+
+    resources = [
+        Resource(capacity, idx < num_renewable)  # resources are ordered
+        for idx, capacity in enumerate(capacities)
+    ]
+    activities = []
+
+    for idx in range(num_activities):
+        duration, *demands = map(int, next(lines).split())
+        line = map(int, next(lines).split())
+
+        groups = []
+        num_groups = next(line)
+        for _ in range(num_groups):
+            num_successors = next(line)
+            groups.append([next(line) for _ in range(num_successors)])
+
+        num_successors, *successors = map(int, next(lines).split())
+        activities.append(
+            Activity(
+                [Mode(duration, demands)],
+                successors,
+                optional=idx > 0,  # source activity is not optional
+                selection_groups=groups,
+            )
+        )
+
+    project = Project(list(range(num_activities)))
+    return ProjectInstance(resources, activities, [project])
diff --git a/tests/data/aslib0_0.rcp b/tests/data/aslib0_0.rcp
new file mode 100644
index 0000000..d90416f
--- /dev/null
+++ b/tests/data/aslib0_0.rcp
@@ -0,0 +1,251 @@
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diff --git a/tests/data/rcpsp_ps.txt b/tests/data/rcpsp_ps.txt
new file mode 100644
index 0000000..dede5c8
--- /dev/null
+++ b/tests/data/rcpsp_ps.txt
@@ -0,0 +1,546 @@
+136 4 0 
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+
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+
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+
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+
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+
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+
+7 7 9 0 6 
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+
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+
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+1 130
+
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+
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+
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+
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+
+7 7 9 0 6 
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+
+0 0 0 0 0 
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+1 135
+
+0 0 0 0 0 
+0 
+0 
diff --git a/tests/test_parse.py b/tests/test_parse.py
index 7fbd475..00c2fdd 100644
--- a/tests/test_parse.py
+++ b/tests/test_parse.py
@@ -11,6 +11,7 @@
         ("data/RG300_1.rcp", "patterson"),
         ("data/MPLIB1_Set1_0.rcmp", "mplib"),
         ("data/UBO10_01.sch", "rcpsp_max"),
+        ("data/rcpsp_ps.txt", "rcpsp_ps"),
     ],
 )
 def test_parse(loc, instance_format):
diff --git a/tests/test_parse_aslib.py b/tests/test_parse_aslib.py
new file mode 100644
index 0000000..409806f
--- /dev/null
+++ b/tests/test_parse_aslib.py
@@ -0,0 +1,42 @@
+from numpy.testing import assert_equal
+
+from psplib import parse_aslib
+
+from .utils import relative
+
+
+def test_aslib0_0():
+    """
+    Tests that the instance ``aslib0_0.rcp`` is correctly parsed.
+    """
+    instance = parse_aslib(relative("data/aslib0_0.rcp"))
+    assert_equal(instance.num_resources, 5)
+
+    capacities = [res.capacity for res in instance.resources]
+    renewables = [res.renewable for res in instance.resources]
+
+    assert_equal(capacities, [10, 10, 10, 10, 10])
+    assert_equal(renewables, [True, True, True, True, True])
+
+    assert_equal(instance.num_activities, 122)
+
+    activity = instance.activities[0]  # source
+    assert_equal(activity.successors, [1, 13, 25, 37, 49])
+    assert_equal(activity.optional, False)  # source always present
+    assert_equal(activity.selection_groups, [[1, 13, 25, 37, 49]])
+
+    assert_equal(activity.num_modes, 1)
+    assert_equal(activity.modes[0].demands, [0, 0, 0, 0, 0])
+    assert_equal(activity.modes[0].duration, 0)
+
+    activity = instance.activities[2]
+    assert_equal(activity.successors, [9, 8])
+    assert_equal(activity.optional, True)
+    assert_equal(activity.selection_groups, [[9], [8]])
+
+    assert_equal(activity.num_modes, 1)
+    assert_equal(activity.modes[0].demands, [0, 0, 1, 0, 0])
+    assert_equal(activity.modes[0].duration, 1)
+
+    assert_equal(instance.num_projects, 1)
+    assert_equal(instance.projects[0].num_activities, 122)
diff --git a/tests/test_parse_mplib.py b/tests/test_parse_mplib.py
index 3fc9738..1ce5998 100644
--- a/tests/test_parse_mplib.py
+++ b/tests/test_parse_mplib.py
@@ -18,11 +18,6 @@ def test_mplib_set1():
     assert_equal(capacities, [56, 56, 56, 56])
     assert_equal(renewables, [True, True, True, True])
 
-    assert_equal(instance.num_projects, 6)
-    for project in instance.projects:
-        assert_equal(project.num_activities, 62)
-        assert_equal(project.release_date, 0)
-
     assert_equal(instance.num_activities, 6 * 62)
 
     activity = instance.activities[0]
@@ -35,6 +30,11 @@ def test_mplib_set1():
     assert_equal(activity.modes[0].demands, [0, 0, 0, 0])
     assert_equal(activity.modes[0].duration, 0)
 
+    assert_equal(instance.num_projects, 6)
+    for project in instance.projects:
+        assert_equal(project.num_activities, 62)
+        assert_equal(project.release_date, 0)
+
 
 def test_mplib_set2():
     """
@@ -49,11 +49,6 @@ def test_mplib_set2():
     assert_equal(capacities, [48, 48, 46, 50, 48])
     assert_equal(renewables, [True, True, True, True, True])
 
-    assert_equal(instance.num_projects, 10)
-    for project in instance.projects:
-        assert_equal(project.num_activities, 52)
-        assert_equal(project.release_date, 0)
-
     assert_equal(instance.num_activities, 10 * 52)
 
     activity = instance.activities[-51]  # second activity of last project
@@ -66,3 +61,8 @@ def test_mplib_set2():
     assert_equal(activity.num_modes, 1)
     assert_equal(activity.modes[0].demands, [8, 4, 3, 5, 1])
     assert_equal(activity.modes[0].duration, 7)
+
+    assert_equal(instance.num_projects, 10)
+    for project in instance.projects:
+        assert_equal(project.num_activities, 52)
+        assert_equal(project.release_date, 0)
diff --git a/tests/test_parse_patterson.py b/tests/test_parse_patterson.py
index dc33c82..5fd06c0 100644
--- a/tests/test_parse_patterson.py
+++ b/tests/test_parse_patterson.py
@@ -18,8 +18,6 @@ def test_rg300():
     assert_equal(capacities, [10, 10, 10, 10])
     assert_equal(renewables, [True, True, True, True])
 
-    assert_equal(instance.num_projects, 1)
-    assert_equal(instance.projects[0].num_activities, 302)
     assert_equal(instance.num_activities, 302)
 
     activity = instance.activities[1]  # second activity
@@ -36,3 +34,6 @@ def test_rg300():
     assert_equal(activity.num_modes, 1)
     assert_equal(activity.modes[0].demands, [0, 1, 0, 0])
     assert_equal(activity.modes[0].duration, 3)
+
+    assert_equal(instance.num_projects, 1)
+    assert_equal(instance.projects[0].num_activities, 302)
diff --git a/tests/test_parse_psplib.py b/tests/test_parse_psplib.py
index 7d1b986..c0ee3e5 100644
--- a/tests/test_parse_psplib.py
+++ b/tests/test_parse_psplib.py
@@ -18,8 +18,6 @@ def test_instance_single_mode():
     assert_equal(capacities, [12, 9, 37, 53])
     assert_equal(renewables, [True, True, False, False])
 
-    assert_equal(instance.num_projects, 1)
-    assert_equal(instance.projects[0].num_activities, 18)
     assert_equal(instance.num_activities, 18)
 
     activity = instance.activities[1]  # second activity (jobnr. 2)
@@ -31,6 +29,9 @@ def test_instance_single_mode():
     assert_equal(activity.modes[0].duration, 2)
     assert_equal(activity.modes[0].demands, [0, 4, 8, 0])
 
+    assert_equal(instance.num_projects, 1)
+    assert_equal(instance.projects[0].num_activities, 18)
+
 
 def test_instance_mmlib():
     """
@@ -46,9 +47,6 @@ def test_instance_mmlib():
     assert_equal(capacities, [33, 33, 247, 248])
     assert_equal(renewables, [True, True, False, False])
 
-    assert_equal(instance.num_projects, 1)
-    assert_equal(instance.projects[0].num_activities, 52)
-
     assert_equal(instance.num_activities, 52)
 
     activity = instance.activities[1]  # second activity (jobnr. 2)
@@ -63,3 +61,6 @@ def test_instance_mmlib():
     assert_equal(activity.modes[1].demands, [5, 5, 2, 6])
     assert_equal(activity.modes[2].duration, 4)
     assert_equal(activity.modes[2].demands, [4, 5, 2, 6])
+
+    assert_equal(instance.num_projects, 1)
+    assert_equal(instance.projects[0].num_activities, 52)
diff --git a/tests/test_parse_rcpsp_max.py b/tests/test_parse_rcpsp_max.py
index 41512e5..e7637f6 100644
--- a/tests/test_parse_rcpsp_max.py
+++ b/tests/test_parse_rcpsp_max.py
@@ -18,15 +18,15 @@ def test_ubo10():
     assert_equal(capacities, [10, 10, 10, 10, 10])
     assert_equal(renewables, [True, True, True, True, True])
 
-    assert_equal(instance.num_projects, 1)
-    assert_equal(instance.projects[0].num_activities, 12)
     assert_equal(instance.num_activities, 12)
 
     activity = instance.activities[2]  # third activity
-
     assert_equal(activity.successors, [4, 11, 7])
     assert_equal(activity.delays, [5, 9, 0])
 
     assert_equal(activity.num_modes, 1)
     assert_equal(activity.modes[0].demands, [10, 8, 0, 8, 10])
     assert_equal(activity.modes[0].duration, 9)
+
+    assert_equal(instance.num_projects, 1)
+    assert_equal(instance.projects[0].num_activities, 12)
diff --git a/tests/test_parse_rcpsp_ps.py b/tests/test_parse_rcpsp_ps.py
new file mode 100644
index 0000000..9def29c
--- /dev/null
+++ b/tests/test_parse_rcpsp_ps.py
@@ -0,0 +1,42 @@
+from numpy.testing import assert_equal
+
+from psplib import parse_rcpsp_ps
+
+from .utils import relative
+
+
+def test_rcpsp_ps():
+    """
+    Tests that the instance ``rcpsp_ps.txt`` is correctly parsed.
+    """
+    instance = parse_rcpsp_ps(relative("data/rcpsp_ps.txt"))
+
+    assert_equal(instance.num_resources, 4)
+
+    capacities = [res.capacity for res in instance.resources]
+    renewables = [res.renewable for res in instance.resources]
+
+    assert_equal(capacities, [10, 10, 10, 10])
+    assert_equal(renewables, [True, True, True, True])
+
+    assert_equal(instance.num_activities, 136)
+
+    activity = instance.activities[0]  # first activity (source)
+    assert_equal(activity.successors, [1, 2])
+    assert_equal(activity.optional, False)  # source always present
+    assert_equal(activity.selection_groups, [[1, 2]])
+
+    assert_equal(activity.num_modes, 1)
+    assert_equal(activity.modes[0].demands, [0, 0, 0, 0])
+    assert_equal(activity.modes[0].duration, 0)
+
+    activity = instance.activities[3]  # fourth activity
+    assert_equal(activity.successors, [5])
+    assert_equal(activity.selection_groups, [[5]])
+
+    assert_equal(activity.num_modes, 1)
+    assert_equal(activity.modes[0].demands, [0, 0, 1, 1])
+    assert_equal(activity.modes[0].duration, 9)
+
+    assert_equal(instance.num_projects, 1)
+    assert_equal(instance.projects[0].num_activities, 136)