I have this problem that while program options works just fine for processing registered options, an unregistered option triggers a MS runtime exception (I'm using MinGW, so it uses MS runtime) that can't be caught at the C++ level. My store() function is shown below: po::store(po::command_line_parser(argc, argv). options(all_verbose). positional(pd). run(), vm); I saw the allow_unregistered() feature. I thought this would be a way to get all illegal command line options "out of the way" so they couldn't trigger the exception. So I modified the above to be: po::store(po::command_line_parser(argc, argv). options(all_verbose). positional(pd). allow_unregistered(). run(), vm); However, putting an illegal argument on the command line, say --x, still gives the message, "This application has requested the Runtime to terminate it in an unusual way." Is an unregistered option triggering some sort of NULL access? Merrill