diff --git a/CHANGELOG.md b/CHANGELOG.md
index d9f22018..c5e2ab0a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 * [#118](https://github.com/stlehmann/pyads/pull/118) Add support for arrays in notification decorator
 
 ### Changed
+* [#128](https://github.com/stlehmann/pyads/pull/128) Deprecation warning for older non-class functions. In
+future versions only methods of the Connection class are supported.
 
 ### Removed
 * [#127](https://github.com/stlehmann/pyads/pull/127) Drop support for Python 2
diff --git a/pyads/ads.py b/pyads/ads.py
index 5a786286..b893937b 100644
--- a/pyads/ads.py
+++ b/pyads/ads.py
@@ -12,7 +12,7 @@
 from ctypes import memmove, addressof, c_ubyte, Array, Structure, sizeof
 from collections import OrderedDict
 
-from .utils import platform_is_linux
+from .utils import platform_is_linux, deprecated
 from .filetimes import filetime_to_dt
 
 from .pyads_ex import (
@@ -162,6 +162,7 @@ def set_local_address(ams_netid):
         )  # pragma: no cover
 
 
+@deprecated()
 def read_state(adr):
     # type: (AmsAddr) -> Optional[Tuple[int, int]]
     """Read the current ADS-state and the machine-state.
@@ -180,6 +181,7 @@ def read_state(adr):
     return None
 
 
+@deprecated()
 def write_control(adr, ads_state, device_state, data, plc_datatype):
     # type: (AmsAddr, int, int, Any, Type) -> None
     """Change the ADS state and the machine-state of the ADS-server.
@@ -206,6 +208,7 @@ def write_control(adr, ads_state, device_state, data, plc_datatype):
         )
 
 
+@deprecated()
 def read_device_info(adr):
     # type: (AmsAddr) -> Optional[Tuple[str, AdsVersion]]
     """Read the name and the version number of the ADS-server.
@@ -221,6 +224,7 @@ def read_device_info(adr):
     return None
 
 
+@deprecated()
 def write(adr, index_group, index_offset, value, plc_datatype):
     # type: (AmsAddr, int, int, Any, Type) -> None
     """Send data synchronous to an ADS-device.
@@ -240,6 +244,7 @@ def write(adr, index_group, index_offset, value, plc_datatype):
         )
 
 
+@deprecated()
 def read_write(
     adr,
     index_group,
@@ -286,6 +291,7 @@ def read_write(
     return None
 
 
+@deprecated()
 def read(
     adr, index_group, index_offset, plc_datatype, return_ctypes=False, check_length=True
 ):
@@ -319,6 +325,7 @@ def read(
     return None
 
 
+@deprecated()
 def read_by_name(adr, data_name, plc_datatype, return_ctypes=False, check_length=True):
     # type: (AmsAddr, str, Type, bool) -> Any
     """Read data synchronous from an ADS-device from data name.
@@ -342,6 +349,7 @@ def read_by_name(adr, data_name, plc_datatype, return_ctypes=False, check_length
     return None
 
 
+@deprecated()
 def write_by_name(adr, data_name, value, plc_datatype):
     # type: (AmsAddr, str, Any, Type) -> None
     """Send data synchronous to an ADS-device from data name.
@@ -412,8 +420,9 @@ def delete_route(adr):
     return adsDelRoute(adr.netIdStruct())
 
 
+@deprecated()
 def add_device_notification(adr, data, attr, callback, user_handle=None):
-    # type: (AmsAddr, Union[str, Tuple[int, int], NotificationAttrib, Callable, int) -> Optional[Tuple[int, int]]  # noqa: E501
+    # type: (AmsAddr, Union[str, Tuple[int, int]], NotificationAttrib, Callable, int) -> Optional[Tuple[int, int]]  # noqa: E501
     """Add a device notification.
 
     :param pyads.structs.AmsAddr adr: AMS Address associated with the routing
@@ -440,6 +449,7 @@ def add_device_notification(adr, data, attr, callback, user_handle=None):
     return None
 
 
+@deprecated()
 def del_device_notification(adr, notification_handle, user_handle):
     # type: (AmsAddr, int, int) -> None
     """Remove a device notification.
diff --git a/pyads/utils.py b/pyads/utils.py
index cf299e57..91a574d7 100644
--- a/pyads/utils.py
+++ b/pyads/utils.py
@@ -9,8 +9,9 @@
 :last modified time: 2018-07-12 14:11:12
 
 """
+import functools
 import sys
-from ctypes import c_ubyte
+import warnings
 
 
 def platform_is_linux():
@@ -24,3 +25,24 @@ def platform_is_windows():
     """Return True if current platform is Windows."""
     # cli being .NET (IronPython)
     return sys.platform == "win32" or sys.platform == "cli"
+
+
+def deprecated(message=None):
+    """Decorator for deprecated functions.
+
+    Shows a deprecation warning with the given message if the
+    decorated function is called.
+
+    """
+    if message is None:
+        message = "Deprecated. This function will not be available in future versions."
+
+    def decorator(func):
+        @functools.wraps(func)
+        def wrapper(*args, **kwargs):
+            warnings.warn(message, DeprecationWarning)
+            return func(*args, **kwargs)
+
+        return wrapper
+
+    return decorator
diff --git a/tests/test_utils.py b/tests/test_utils.py
new file mode 100644
index 00000000..976b08f3
--- /dev/null
+++ b/tests/test_utils.py
@@ -0,0 +1,13 @@
+from unittest import TestCase
+
+from pyads.utils import deprecated
+
+
+class UtilsTestCase(TestCase):
+    def test_deprecated_decorator(self):
+        @deprecated()
+        def deprecated_fct():
+            pass
+
+        with self.assertWarns(DeprecationWarning):
+            deprecated_fct()