cxxflags ought to work. You can use -d2 to find out the compiler options that Boost.Build is using.
I'm calling with the following command line: bjam --toolset=msvc-8.0 --link=static --runtime-link=shared --with-thread --cxxflags="/FI\"C:/boost _1_44_0/sxs_header.h\"" -d2 Where I put the extraneous header in the dir listed, and I see the below for each file. Attaching full log. file bin.v2\libs\thread\build\msvc-8.0\debug\link-static\threading-multi\win32\thread.obj.rsp "libs\thread\src\win32\thread.cpp" -Fo"bin.v2\libs\thread\build\msvc-8.0\debug\link-static\threading-multi\win32\thread.obj" -TP /Z7 /Od /Ob0 /W3 /GR /MDd /Zc:forScope /Zc:wchar_t /wd4675 /EHs -c -DBOOST_ALL_NO_LIB=1 -DBOOST_THREAD_BUILD_LIB=1 "-I." compile-c-c++ bin.v2\libs\thread\build\msvc-8.0\debug\link-static\threading-multi\win32\thread.obj call "C:\Program Files (x86)\Microsoft Visual Studio 8\VC\vcvarsall.bat" x86 >nul cl /Zm800 -nologo @"bin.v2\libs\thread\build\msvc-8.0\debug\link-static\threading-multi\win32\thread.obj.rsp" thread.cpp