Hi, I have the same problem here since this morning, as XP installed automatically the .NET 2.0 SP 1 update. Unit test 'leaktest.py' illustrated the problem :: H:\temp\pythonnet\pythonnet_orig\src\tests>..\..\python.exe leaktest.py Running module leak check... start: 16109568 end: 17137664 diff: +1028096 Running class leak check... Unhandled Exception: System.TypeInitializationException: The type initializer for 'Python.Runtime.CodeGenerator' threw an exception. -- -> System.InvalidCastException: Unable to cast object of type 'System.Reflection.Module' to type 'System.Reflection.Emit.ModuleBuilder'. at System.Reflection.Emit.AssemblyBuilderData.GetInMemoryAssemblyModule() at System.AppDomain.InternalDefineDynamicAssembly(AssemblyName name, AssemblyBuilderAccess access, String dir, Evidence evidence, PermissionSet requiredPermissions, PermissionSet optionalPermissions, PermissionSet refusedPermissions, StackCrawlMark& stackMark, IEnumerable`1 unsafeAssemblyAttributes) at System.AppDomain.DefineDynamicAssembly(AssemblyName name, AssemblyBuilderAccess access) at Python.Runtime.CodeGenerator..cctor() in H:\temp\pythonnet\pythonnet_orig\src\runtime\codegenerator.cs:line 37 --- End of inner exception stack trace --- at Python.Runtime.DelegateManager.GetDispatcher(Type dtype) in H:\temp\pythonnet\pythonnet_orig\src\runtime\delegatemanager.cs:line 79 at Python.Runtime.DelegateManager.GetDelegate(Type dtype, IntPtr callable) in H:\temp\pythonnet\pythonnet_orig\src\runtime\delegatemanager.cs:line 168 at Python.Runtime.DelegateObject.tp_new(IntPtr tp, IntPtr args, IntPtr kw) in H:\temp\pythonnet\pythonnet_orig\src\runtime\delegateobject.cs:line 74 at e__NativeCall.Call_3(IntPtr , IntPtr , IntPtr , IntPtr ) at Python.Runtime.MetaType.tp_call(IntPtr tp, IntPtr args, IntPtr kw) in H:\temp\pythonnet\pythonnet_orig\src\runtime\metatype.cs:line 156 at Python.Runtime.Runtime.Py_Main(Int32 argc, String[] argv) at Python.Runtime.PythonConsole.Main(String[] args) in H:\temp\pythonnet\pythonnet_orig\src\console\pythonconsole.cs:line 24 I think, for now, I will uninstall this SP1... Cheers, Nicolas.