Hi, I saw that you don't serialize accesses to member data, so I wouldn't call your class thread-safe but reentrant, since it can be safely instantiated in several concurrent threads, which shouldn't however share the same instance. Maybe I have a wrong idea of what a thread safe class should be.... I use the definitions from Qt: < http://doc.trolltech.com/4.1/threads.html#reentrancy-and-thread-safety> I think that UUID doesn't need synchronization, but it is important that reading the documentation the user cannot misunderstanding the library guarantees against multi-threading. I run on my Linux box (equipped with gcc (GCC) 4.3.0) the tests "test_uuid.cpp" and "test_sha1.cpp" which pass. I didn't run the "test_serialization.cpp" since I use a prepackaged version of boost. Obiuvsly, I run the tests against the source with the modifcations I posted on this mailing list. Best regards, Manuel Fiorelli