Great, Brian. I successfully reproduced the state with a very simple application. Here is very dumb python program that uses the CLR: from CLR.System import String class cSharpCaller( object ): def __init__( self ): print "Created a cSharpCaller instance" self.__cSharpString = None def createCSharpString( self, str ): self.__cSharpString = String( str ) def returnCSharpString( self ): return self.__cSharpString.ToString() And if I test this with this (dumb) threethreaded testprogram: import threading, thread, time from cSharpCaller import cSharpCaller def start_threads(amount=5): for i in range(amount): thread = threading.Thread(target=process_thread ) print "starting thread ", thread.getName() thread.start() def process_thread( ): print "thread started " for i in range( 2 ): print "Hi, I'm a thread" time.sleep( 1 ) cs = cSharpCaller( ) cs.createCSharpString( "created from a thread" ) print "cs.returnCSharpString() = ", cs.returnCSharpString() print "thread %s ended" start_threads( 3 ) I get this output: starting thread thread0 thread started Hi, I'm thread %s starting thread thread1 thread started Hi, I'm thread %s starting thread thread2 thread started Hi, I'm thread %s Hi, I'm thread %s Hi, I'm thread %s Hi, I'm thread %s Created a cSharpCaller instance Created a cSharpCaller instance cs.returnCSharpString() = and I have to CTRL+Pause to get out of the hang. Clearly hangs on the first String.ToString() method call. Regards, Torgeir