Hi, I just had this similar error today, but it took me quite a few minutes to build a working "example". I'll start off with the error(s): L:\project\serializationtest\Mixed.hpp:27: error: conflicting declaration 'const boost::archive::detail::guid_initializer<Mixed>&<unnamed>::boost_serialization_guid_initializer_27' L:\project\serializationtest\Child.hpp:27: error: '<unnamed>::boost_serialization_guid_initializer_27' has a previous declaration as `const boost::archive::detail::guid_initializer<Child>&<unnamed>::boost_serialization_guid_initializer_27' L:\project\serializationtest\Mixed.hpp:27: error: type mismatch with previous external decl of `const boost::archive::detail::guid_initializer<Mixed>&<unnamed>::boost_serialization_guid_initializer_27' L:\project\serializationtest\Child.hpp:27: error: previous external decl of `const boost::archive::detail::guid_initializer<Child>&<unnamed>::boost_serialization_guid_initializer_27' L:\project\serializationtest\Mixed.hpp:27: error: declaration of `const boost::archive::detail::guid_initializer<Mixed>&<unnamed>::boost_serialization_guid_initializer_27' L:\project\serializationtest\Child.hpp:27: error: conflicts with previous declaration `const boost::archive::detail::guid_initializer<Child>&<unnamed>::boost_serialization_guid_initializer_27' :: === Build finished: 6 errors, 0 warnings === This is my class structure: Base Child : Base Mixed : Base or #include "Base.hpp" is actually enough. Easy, but maybe its already a hint to the problem. The "original" Class structure in my app is far more complex, so I just will refer to my little example, as I guess its the same source some how. I tend to believe that the problem resides somewhere in the Macro BOOST_CLASS_EXPORT, as the compiler points to this. I'm working on Windows XP, with MinGW and the latest (Monday) SVN Build of boost. (as serialization is in 1.36 partly broken afaik/imho). I'll attach my code. So, how to resolve this? regards Jens Weller