Hi there, my project is using its own created zlib.lib, thats why I want to create the iostreams libs using our own zlib.lib . Creating the iostreams libs I use the following build command: bjam -sTOOLS=vc-7_1 --libdir=c:\boost\lib --with-iostreams "-sZLIB_BINARY=C:\CHH\External\Libraries\Release\zlib" "-sZLIB_INCLUDE=C:\CHH\External\Includes\zlib" "-sZLIB_LIBPATH=C:\CHH\External\Libraries\Release" --without-python stage Note that the ZLIB_SOURCE variable is missing. Otherwise the build would create it's own zlib library. Here is the warning I get when bjam is building the debug lib: LINK : warning LNK4098: defaultlib 'MSVCRT' conflicts with use of other libs; use /NODEFAULTLIB:library The problem arises because it's using a release built zlib.lib with the debug built iostream lib, since I can only pass one zlib to the build command. A solution would be to have two variables ZLIB_LIBPATH_DEBUG and ZLIB_LIBPATH_RELEASE. Which is the standard procedure. Or is there a variable in the boost build for creating debug lib only? Greets, Christian