Robert, The immediate object being serialized was a shared_ptr and one of the data members being serialized was also a shared_ptr, although it doesn't appear that the process got to the point of serializing that data member. Removing the data member from serialization did nothing. The same faulty output was created. Removing the shared_ptr data member AND replacing the testObj_out shared_ptr with an ordinary pointer produced a different mal-formed XML output: <?xml version="1.0" encoding="UTF-8" standalone="yes" ?> <!DOCTYPE boost_serialization> <boost_serialization signature="serialization::archive" version="3"> <testObj_out class_id="0" tracking_level="1" version="0" object_id="_0"</boost_serialization> By the way, in your reply, you mentioned the missing </px element. However, the original error (and this one too) is more primitive: the initial <px element (or, in this case, the <testObj_out element) have no closing >. Therefore, the serialization process never got past outputting the FULL introductory element. I doubt that the processing ever got to the point of serializing any of the data members. Merrill