Another observation: the difference between g++/Sun in ST and MT mode: http://www.boost.org/development/tests/trunk/developer/output/Sandia-sun-boo... http://www.boost.org/development/tests/trunk/developer/output/Sandia-sun-boo... is in the _PTHREADS macro (_REENTRANT is defined in both cases, which causes BOOST_HAS_THREADS and BOOST_HAS_PTHREADS to be defined as well even in ST mode.) Taking a Linux g++ at random: http://www.boost.org/development/tests/trunk/developer/output/Sandia-gcc-boo... http://www.boost.org/development/tests/trunk/developer/output/Sandia-gcc-boo... _REENTRANT is correctly only defined in MT mode, but BOOST_HAS_THREADS (and BOOST_HAS_PTHREADS) incorrectly remains defined in both. g++ 4.1 also seems to work the same way, whereas g++ 4.0 and g++ 3.4 do not, they always define _REENTRANT and there doesn't seem to be a way to detect MT mode (at least judging by the output of config_info).