I think the following example serves better for understanding my problem. I don't think that at_key is the right way to go. Instead, I'm looking for something like that finds me the right key for a value. #include <boost/fusion/sequence.hpp> using namespace std; using namespace boost; namespace fields { struct id {}; struct name {}; struct address {}; struct birthday {}; } namespace tags { struct pri {}; struct val {}; } typedef fusion::map< fusion::pair< fields::id, tags::pri > , fusion::pair< fields::name, tags::val > , fusion::pair< fields::address, tags::val > , fusion::pair< fields::birthday, tags::val > > table; int _tmain(int argc, _TCHAR* argv[]) { BOOST_STATIC_ASSERT((is_same< fusion::result_of::???< table , tags::pri >::type , fields::id >::value )); return 0; } Does that makes sense, now? Christian