I upgraded from boost 1.48 to .154 and I thought I was done after all the compiler errors and unit tests were ironed out. But now when I start our application, it terminates immediately. This only happens with the debug build. The release build is fine. I verified that it properly links to boost-python-vc100-mt-gyd-1_54.lib and python27_d.lib The python version (2.73) is still the same that we used with boost 1.48. I tracked it down to here: Py_Initialize(); m_mainModule = boost::python::import(__main__); <-- crash here With the debugger I found out that the string object is invalid. More specifically, ob_type is 0xffffffff. So I added this exception which always throws now: Py_Initialize(); const boost::pathon::str someRandomString("someRandomString"); if(reinterpret_cast<void*>(0xffffffff) == someRandomString.ptr()->ob_type) throw std::runtime_error("Python problem"); What could be going on here? Rgds Richard