In following up the attached message the cause appears to be a Boost.Build issue because specifying: bjam --toolset=gcc-nocygwin has no effect on the gcc command line, the generated command lines are simply: "g++" -ftemplate-depth-128 -O0 -fno-inline -Wall -g -mthreads -DBOOST_ALL_NO_LIB=1 -DBOOST_REGEX_DYN_LINK=1 -I"." -c -o "bin.v2\libs\regex\build\gcc-nocygwin\debug\threading-multi\cpp_regex_traits.o" "libs\regex\src\..\src\cpp_regex_traits.cpp" So there's no -mno-cygwin flag and the binaries are actually built for cygwin not mingw. At the same time there is no documented means of building with Mingw in the getting started guide: should there be? Cheers, John.