diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index b69b506..d7348e8 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -83,6 +83,11 @@ jobs:
           pipx run --python '${{ steps.setup-python.outputs.python-path }}'
           tox --installpkg '${{ needs.prepare.outputs.wheel-distribution }}'
           -- -rFEx --durations 10 --color yes  # pytest args
+      - name: Run tests (s2)
+        run: >-
+          pipx run --python '${{ steps.setup-python.outputs.python-path }}'
+          tox -e s2 --installpkg '${{ needs.prepare.outputs.wheel-distribution }}'
+          -- -rFEx --durations 10 --color yes  # pytest args
       # - name: Generate coverage report
       #   run: pipx run coverage lcov -o coverage.lcov
       # - name: Upload partial coverage report
diff --git a/README.rst b/README.rst
index cc60f9c..035f3da 100644
--- a/README.rst
+++ b/README.rst
@@ -56,6 +56,10 @@ Install using ``pip``::
 
     pip install flexmeasures-client
 
+To enable S2 features, you need to install extra requirements::
+
+    pip install flexmeasures-client[s2]
+
 Initialization and authentication::
 
     from flexmeasures_client import FlexMeasuresClient
@@ -144,6 +148,13 @@ If you want to develop this package it's necessary to install testing requiremen
 
     pip install -e ".[testing]"
 
+Moreover, if you need to work on S2 features, you need to install extra dependencies::
+
+    pip install -e ".[s2, testing]"
+
+
+
+
 .. _pyscaffold-notes:
 
 
@@ -163,6 +174,9 @@ Running tests locally is crucial as well. Staying close to the CI workflow::
     tox -e clean,build
     tox -- -rFEx --durations 10 --color yes
 
+For S2 features, you need to add `-e s2` to tox::
+
+    tox -e s2
 
 This project uses `pre-commit`_, please make sure to install it before making any
 changes::
diff --git a/pytest.ini b/pytest.ini
new file mode 100644
index 0000000..820dca4
--- /dev/null
+++ b/pytest.ini
@@ -0,0 +1,2 @@
+[pytest]
+addopts = --ignore=tests/s2
diff --git a/setup.cfg b/setup.cfg
index 7583f47..7417c6f 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -51,7 +51,6 @@ install_requires =
     importlib-metadata; python_version<"3.8"
     aiohttp
     pandas>=2.1.4
-    s2-python>=0.4.1 # reason: be compatible with constraint pydantic>=2.8.2
     async_timeout
 
 [options.packages.find]
@@ -61,8 +60,8 @@ exclude =
 
 [options.extras_require]
 # Add here additional requirements for extra features, to install with:
-# `pip install flexmeasures-client[PDF]` like:
-# PDF = ReportLab; RXP
+# `pip install flexmeasures-client[s2]` like:
+s2=s2-python>=0.4.1 # reason: be compatible with constraint pydantic>=2.8.2
 
 # Add here test requirements (semicolon/line-separated)
 testing =
@@ -73,7 +72,6 @@ testing =
     pytest-mock
     aioresponses
 
-
 [options.entry_points]
 # Add here console scripts like:
 # console_scripts =
@@ -85,6 +83,7 @@ testing =
 # pyscaffold.cli =
 #     awesome = pyscaffoldext.awesome.extension:AwesomeExtension
 
+
 [tool:pytest]
 # Specify command line options as you would do when invoking pytest directly.
 # e.g. --cov-report html (or xml) for html/xml output or --junitxml junit.xml
diff --git a/src/flexmeasures_client/s2/__init__.py b/src/flexmeasures_client/s2/__init__.py
index 66117f5..5b90b45 100644
--- a/src/flexmeasures_client/s2/__init__.py
+++ b/src/flexmeasures_client/s2/__init__.py
@@ -7,7 +7,15 @@
 from typing import Callable, Coroutine, Dict, Type
 
 import pydantic
-from s2python.common import ReceptionStatus, ReceptionStatusValues, RevokeObject
+
+try:
+    from s2python.common import ReceptionStatus, ReceptionStatusValues, RevokeObject
+except ImportError:
+    raise ImportError(
+        "The 's2-python' package is required for this functionality. "
+        "Install it using `pip install flexmeasures-client[s2]`."
+    )
+
 
 from flexmeasures_client.s2.utils import (
     SizeLimitOrderedDict,
diff --git a/src/flexmeasures_client/s2/cem.py b/src/flexmeasures_client/s2/cem.py
index 8d40280..4d3fb4e 100644
--- a/src/flexmeasures_client/s2/cem.py
+++ b/src/flexmeasures_client/s2/cem.py
@@ -7,17 +7,25 @@
 from typing import Dict, Optional
 
 import pydantic
-from s2python.common import (
-    ControlType,
-    Handshake,
-    HandshakeResponse,
-    PowerMeasurement,
-    ReceptionStatus,
-    ReceptionStatusValues,
-    ResourceManagerDetails,
-    RevokeObject,
-    SelectControlType,
-)
+
+try:
+    from s2python.common import (
+        ControlType,
+        Handshake,
+        HandshakeResponse,
+        PowerMeasurement,
+        ReceptionStatus,
+        ReceptionStatusValues,
+        ResourceManagerDetails,
+        RevokeObject,
+        SelectControlType,
+    )
+except ImportError:
+    raise ImportError(
+        "The 's2-python' package is required for this functionality. "
+        "Install it using `pip install flexmeasures-client[s2]`."
+    )
+
 
 from flexmeasures_client.client import FlexMeasuresClient
 from flexmeasures_client.s2 import Handler, register
diff --git a/src/flexmeasures_client/s2/control_types/FRBC/__init__.py b/src/flexmeasures_client/s2/control_types/FRBC/__init__.py
index ad2b8c3..2b4ccf1 100644
--- a/src/flexmeasures_client/s2/control_types/FRBC/__init__.py
+++ b/src/flexmeasures_client/s2/control_types/FRBC/__init__.py
@@ -1,17 +1,25 @@
 import asyncio
 
 import pydantic
-from s2python.common import ControlType, ReceptionStatusValues
-from s2python.frbc import (
-    FRBCActuatorStatus,
-    FRBCFillLevelTargetProfile,
-    FRBCInstruction,
-    FRBCLeakageBehaviour,
-    FRBCStorageStatus,
-    FRBCSystemDescription,
-    FRBCTimerStatus,
-    FRBCUsageForecast,
-)
+
+try:
+    from s2python.common import ControlType, ReceptionStatusValues
+    from s2python.frbc import (
+        FRBCActuatorStatus,
+        FRBCFillLevelTargetProfile,
+        FRBCInstruction,
+        FRBCLeakageBehaviour,
+        FRBCStorageStatus,
+        FRBCSystemDescription,
+        FRBCTimerStatus,
+        FRBCUsageForecast,
+    )
+except ImportError:
+    raise ImportError(
+        "The 's2-python' package is required for this functionality. "
+        "Install it using `pip install flexmeasures-client[s2]`."
+    )
+
 
 from flexmeasures_client.s2 import SizeLimitOrderedDict, register
 from flexmeasures_client.s2.control_types import ControlTypeHandler
diff --git a/src/flexmeasures_client/s2/control_types/FRBC/frbc_simple.py b/src/flexmeasures_client/s2/control_types/FRBC/frbc_simple.py
index 896c281..dcb66c7 100644
--- a/src/flexmeasures_client/s2/control_types/FRBC/frbc_simple.py
+++ b/src/flexmeasures_client/s2/control_types/FRBC/frbc_simple.py
@@ -6,7 +6,19 @@
 from datetime import datetime, timedelta
 
 import pytz
-from s2python.frbc import FRBCActuatorStatus, FRBCStorageStatus, FRBCSystemDescription
+
+try:
+    from s2python.frbc import (
+        FRBCActuatorStatus,
+        FRBCStorageStatus,
+        FRBCSystemDescription,
+    )
+except ImportError:
+    raise ImportError(
+        "The 's2-python' package is required for this functionality. "
+        "Install it using `pip install flexmeasures-client[s2]`."
+    )
+
 
 from flexmeasures_client.s2.control_types.FRBC import FRBC
 from flexmeasures_client.s2.control_types.FRBC.utils import fm_schedule_to_instructions
diff --git a/src/flexmeasures_client/s2/control_types/FRBC/frbc_tunes.py b/src/flexmeasures_client/s2/control_types/FRBC/frbc_tunes.py
index e4b64ce..889e73b 100644
--- a/src/flexmeasures_client/s2/control_types/FRBC/frbc_tunes.py
+++ b/src/flexmeasures_client/s2/control_types/FRBC/frbc_tunes.py
@@ -10,15 +10,23 @@
 import pandas as pd
 import pydantic
 import pytz
-from s2python.common import NumberRange, ReceptionStatus, ReceptionStatusValues
-from s2python.frbc import (
-    FRBCActuatorStatus,
-    FRBCFillLevelTargetProfile,
-    FRBCInstruction,
-    FRBCStorageStatus,
-    FRBCSystemDescription,
-    FRBCUsageForecast,
-)
+
+try:
+    from s2python.common import NumberRange, ReceptionStatus, ReceptionStatusValues
+    from s2python.frbc import (
+        FRBCActuatorStatus,
+        FRBCFillLevelTargetProfile,
+        FRBCInstruction,
+        FRBCStorageStatus,
+        FRBCSystemDescription,
+        FRBCUsageForecast,
+    )
+except ImportError:
+    raise ImportError(
+        "The 's2-python' package is required for this functionality. "
+        "Install it using `pip install flexmeasures-client[s2]`."
+    )
+
 
 from flexmeasures_client.s2 import register
 from flexmeasures_client.s2.control_types.FRBC import FRBC
diff --git a/src/flexmeasures_client/s2/control_types/FRBC/utils.py b/src/flexmeasures_client/s2/control_types/FRBC/utils.py
index 22ea836..1b732c9 100644
--- a/src/flexmeasures_client/s2/control_types/FRBC/utils.py
+++ b/src/flexmeasures_client/s2/control_types/FRBC/utils.py
@@ -3,7 +3,15 @@
 from typing import List
 
 import pandas as pd
-from s2python.frbc import FRBCInstruction, FRBCOperationMode, FRBCSystemDescription
+
+try:
+    from s2python.frbc import FRBCInstruction, FRBCOperationMode, FRBCSystemDescription
+except ImportError:
+    raise ImportError(
+        "The 's2-python' package is required for this functionality. "
+        "Install it using `pip install flexmeasures-client[s2]`."
+    )
+
 
 from flexmeasures_client.s2.utils import get_unique_id
 
diff --git a/src/flexmeasures_client/s2/control_types/translations.py b/src/flexmeasures_client/s2/control_types/translations.py
index db22bcb..87f6ade 100644
--- a/src/flexmeasures_client/s2/control_types/translations.py
+++ b/src/flexmeasures_client/s2/control_types/translations.py
@@ -4,11 +4,18 @@
 
 import numpy as np
 import pandas as pd
-from s2python.frbc import (
-    FRBCFillLevelTargetProfile,
-    FRBCLeakageBehaviour,
-    FRBCUsageForecast,
-)
+
+try:
+    from s2python.frbc import (
+        FRBCFillLevelTargetProfile,
+        FRBCLeakageBehaviour,
+        FRBCUsageForecast,
+    )
+except ImportError:
+    raise ImportError(
+        "The 's2-python' package is required for this functionality. "
+        "Install it using `pip install flexmeasures-client[s2]`."
+    )
 
 
 def leakage_behaviour_to_storage_efficieny(
diff --git a/src/flexmeasures_client/s2/utils.py b/src/flexmeasures_client/s2/utils.py
index 42f0347..12cc9e7 100644
--- a/src/flexmeasures_client/s2/utils.py
+++ b/src/flexmeasures_client/s2/utils.py
@@ -5,7 +5,15 @@
 from uuid import uuid4
 
 import pydantic
-from s2python.common import ReceptionStatus, ReceptionStatusValues
+
+try:
+    from s2python.common import ReceptionStatus, ReceptionStatusValues
+except ImportError:
+    raise ImportError(
+        "The 's2-python' package is required for this functionality. "
+        "Install it using `pip install flexmeasures-client[s2]`."
+    )
+
 
 KT = TypeVar("KT")
 VT = TypeVar("VT")
diff --git a/src/flexmeasures_client/s2/wrapper.py b/src/flexmeasures_client/s2/wrapper.py
index 0a4d9c0..97acf28 100644
--- a/src/flexmeasures_client/s2/wrapper.py
+++ b/src/flexmeasures_client/s2/wrapper.py
@@ -1,7 +1,14 @@
 from datetime import datetime
 
 from pydantic import BaseModel, Field
-from s2python.message import S2Message
+
+try:
+    from s2python.message import S2Message
+except ImportError:
+    raise ImportError(
+        "The 's2-python' package is required for this functionality. "
+        "Install it using `pip install flexmeasures-client[s2]`."
+    )
 
 
 class MetaData(BaseModel):
diff --git a/tests/conftest.py b/tests/conftest.py
index 7480690..e69de29 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -1,121 +0,0 @@
-from __future__ import annotations
-
-from datetime import datetime, timezone
-
-import pytest
-from s2python.common import (
-    Commodity,
-    CommodityQuantity,
-    ControlType,
-    Duration,
-    EnergyManagementRole,
-    Handshake,
-    NumberRange,
-    PowerRange,
-    ResourceManagerDetails,
-    Role,
-    RoleType,
-)
-from s2python.frbc import (
-    FRBCActuatorDescription,
-    FRBCOperationMode,
-    FRBCOperationModeElement,
-    FRBCStorageDescription,
-    FRBCSystemDescription,
-)
-
-from flexmeasures_client.s2.utils import get_unique_id
-
-
-@pytest.fixture(scope="session")
-def frbc_system_description():
-    ########
-    # FRBC #
-    ########
-
-    thp_operation_mode_element = FRBCOperationModeElement(
-        fill_level_range=NumberRange(start_of_range=0, end_of_range=80),
-        fill_rate=NumberRange(start_of_range=0, end_of_range=2),
-        power_ranges=[
-            PowerRange(
-                start_of_range=10,
-                end_of_range=1000,
-                commodity_quantity=CommodityQuantity.ELECTRIC_POWER_3_PHASE_SYMMETRIC,
-            )
-        ],
-    )
-
-    thp_operation_mode = FRBCOperationMode(
-        id=get_unique_id(),
-        elements=[thp_operation_mode_element],
-        abnormal_condition_only=False,
-    )
-
-    nes_operation_mode_element = FRBCOperationModeElement(
-        fill_level_range=NumberRange(start_of_range=0, end_of_range=100),
-        fill_rate=NumberRange(start_of_range=0, end_of_range=1),
-        power_ranges=[
-            PowerRange(
-                start_of_range=10,
-                end_of_range=1000,
-                commodity_quantity=CommodityQuantity.ELECTRIC_POWER_3_PHASE_SYMMETRIC,
-            )
-        ],
-    )
-
-    nes_operation_mode = FRBCOperationMode(
-        id=get_unique_id(),
-        elements=[nes_operation_mode_element],
-        abnormal_condition_only=False,
-    )
-
-    actuator = FRBCActuatorDescription(
-        id=get_unique_id(),
-        supported_commodities=[Commodity.ELECTRICITY],
-        operation_modes=[thp_operation_mode, nes_operation_mode],
-        transitions=[],
-        timers=[],
-    )
-
-    storage = FRBCStorageDescription(
-        provides_leakage_behaviour=True,
-        provides_fill_level_target_profile=True,
-        provides_usage_forecast=True,
-        fill_level_range=NumberRange(start_of_range=0, end_of_range=1),
-    )
-
-    system_description_message = FRBCSystemDescription(
-        message_id=get_unique_id(),
-        valid_from=datetime(2024, 1, 1, tzinfo=timezone.utc),
-        actuators=[actuator],
-        storage=storage,
-    )
-
-    return system_description_message
-
-
-@pytest.fixture(scope="session")
-def resource_manager_details():
-    return ResourceManagerDetails(
-        message_id=get_unique_id(),
-        resource_id=get_unique_id(),
-        roles=[Role(role=RoleType.ENERGY_STORAGE, commodity=Commodity.ELECTRICITY)],
-        instruction_processing_delay=Duration(1),
-        available_control_types=[
-            ControlType.FILL_RATE_BASED_CONTROL,
-            ControlType.NO_SELECTION,
-        ],
-        provides_forecast=True,
-        provides_power_measurement_types=[
-            CommodityQuantity.ELECTRIC_POWER_3_PHASE_SYMMETRIC
-        ],
-    )
-
-
-@pytest.fixture(scope="session")
-def rm_handshake():
-    return Handshake(
-        message_id=get_unique_id(),
-        role=EnergyManagementRole.RM,
-        supported_protocol_versions=["1.0.0"],
-    )
diff --git a/tests/s2/__init__.py b/tests/s2/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/s2/conftest.py b/tests/s2/conftest.py
new file mode 100644
index 0000000..7480690
--- /dev/null
+++ b/tests/s2/conftest.py
@@ -0,0 +1,121 @@
+from __future__ import annotations
+
+from datetime import datetime, timezone
+
+import pytest
+from s2python.common import (
+    Commodity,
+    CommodityQuantity,
+    ControlType,
+    Duration,
+    EnergyManagementRole,
+    Handshake,
+    NumberRange,
+    PowerRange,
+    ResourceManagerDetails,
+    Role,
+    RoleType,
+)
+from s2python.frbc import (
+    FRBCActuatorDescription,
+    FRBCOperationMode,
+    FRBCOperationModeElement,
+    FRBCStorageDescription,
+    FRBCSystemDescription,
+)
+
+from flexmeasures_client.s2.utils import get_unique_id
+
+
+@pytest.fixture(scope="session")
+def frbc_system_description():
+    ########
+    # FRBC #
+    ########
+
+    thp_operation_mode_element = FRBCOperationModeElement(
+        fill_level_range=NumberRange(start_of_range=0, end_of_range=80),
+        fill_rate=NumberRange(start_of_range=0, end_of_range=2),
+        power_ranges=[
+            PowerRange(
+                start_of_range=10,
+                end_of_range=1000,
+                commodity_quantity=CommodityQuantity.ELECTRIC_POWER_3_PHASE_SYMMETRIC,
+            )
+        ],
+    )
+
+    thp_operation_mode = FRBCOperationMode(
+        id=get_unique_id(),
+        elements=[thp_operation_mode_element],
+        abnormal_condition_only=False,
+    )
+
+    nes_operation_mode_element = FRBCOperationModeElement(
+        fill_level_range=NumberRange(start_of_range=0, end_of_range=100),
+        fill_rate=NumberRange(start_of_range=0, end_of_range=1),
+        power_ranges=[
+            PowerRange(
+                start_of_range=10,
+                end_of_range=1000,
+                commodity_quantity=CommodityQuantity.ELECTRIC_POWER_3_PHASE_SYMMETRIC,
+            )
+        ],
+    )
+
+    nes_operation_mode = FRBCOperationMode(
+        id=get_unique_id(),
+        elements=[nes_operation_mode_element],
+        abnormal_condition_only=False,
+    )
+
+    actuator = FRBCActuatorDescription(
+        id=get_unique_id(),
+        supported_commodities=[Commodity.ELECTRICITY],
+        operation_modes=[thp_operation_mode, nes_operation_mode],
+        transitions=[],
+        timers=[],
+    )
+
+    storage = FRBCStorageDescription(
+        provides_leakage_behaviour=True,
+        provides_fill_level_target_profile=True,
+        provides_usage_forecast=True,
+        fill_level_range=NumberRange(start_of_range=0, end_of_range=1),
+    )
+
+    system_description_message = FRBCSystemDescription(
+        message_id=get_unique_id(),
+        valid_from=datetime(2024, 1, 1, tzinfo=timezone.utc),
+        actuators=[actuator],
+        storage=storage,
+    )
+
+    return system_description_message
+
+
+@pytest.fixture(scope="session")
+def resource_manager_details():
+    return ResourceManagerDetails(
+        message_id=get_unique_id(),
+        resource_id=get_unique_id(),
+        roles=[Role(role=RoleType.ENERGY_STORAGE, commodity=Commodity.ELECTRICITY)],
+        instruction_processing_delay=Duration(1),
+        available_control_types=[
+            ControlType.FILL_RATE_BASED_CONTROL,
+            ControlType.NO_SELECTION,
+        ],
+        provides_forecast=True,
+        provides_power_measurement_types=[
+            CommodityQuantity.ELECTRIC_POWER_3_PHASE_SYMMETRIC
+        ],
+    )
+
+
+@pytest.fixture(scope="session")
+def rm_handshake():
+    return Handshake(
+        message_id=get_unique_id(),
+        role=EnergyManagementRole.RM,
+        supported_protocol_versions=["1.0.0"],
+    )
diff --git a/tests/test_cem.py b/tests/s2/test_cem.py
similarity index 100%
rename from tests/test_cem.py
rename to tests/s2/test_cem.py
diff --git a/tests/test_frbc_tunes.py b/tests/s2/test_frbc_tunes.py
similarity index 100%
rename from tests/test_frbc_tunes.py
rename to tests/s2/test_frbc_tunes.py
diff --git a/tests/test_frbc_utils.py b/tests/s2/test_frbc_utils.py
similarity index 100%
rename from tests/test_frbc_utils.py
rename to tests/s2/test_frbc_utils.py
diff --git a/tests/test_s2_models.py b/tests/s2/test_s2_models.py
similarity index 100%
rename from tests/test_s2_models.py
rename to tests/s2/test_s2_models.py
diff --git a/tests/test_s2_translations.py b/tests/s2/test_s2_translations.py
similarity index 100%
rename from tests/test_s2_translations.py
rename to tests/s2/test_s2_translations.py
diff --git a/tox.ini b/tox.ini
index 69f8159..abf99fe 100644
--- a/tox.ini
+++ b/tox.ini
@@ -20,6 +20,12 @@ extras =
 commands =
     pytest {posargs}
 
+[testenv:s2]
+deps =
+    .[s2]  # Install the package with optional dependencies
+commands = pytest tests/s2
+
+
 
 # # To run `tox -e lint` you need to make sure you have a
 # # `.pre-commit-config.yaml` file. See https://pre-commit.com