In config/platform/win32.hpp I see: #if !defined(__GNUC__) && !defined(BOOST_HAS_DECLSPEC) # define BOOST_HAS_DECLSPEC #endif This tells me that __declspec can be possibly used for all Windows compilers except for gcc. I believe, however, that for Comeau-Win32 __declspec can also not be used. When trying to build Boost libraries for Comeau with the VC7.1 backend, the compiler gives this error in the date/time library when trying to compile greg_month.cpp with BOOST_ALL_DYN_LINK defined: ""H:\Utilities\BoostLibraries\boost_1_32_0\boost/date_time/gregorian/greg_month.h pp", line 55: error #70: incomplete type is not allowed class BOOST_DATE_TIME_DECL greg_month : public greg_month_rep {" ^ I could be wrong but this seems to me that the compiler is objecting to BOOST_DATE_TIME_DECL, which becomes __declspec(dllexport) because BOOST_HAS_DECLSPEC is defined for the Comeau compiler. Needless to say many other errors along thr same lines occur in the build for other libraries which use __declspec(dllexport).