The past decade has seen XML widely adopted within a variety of communities, including the digital library community. While it now plays a critical role in the infrastructure of many digital library operations, XML's promise of interoperability of data across systems and organizations has not been fully realized within digital libraries. The reasons for this are not primarily technical in origin, but social, and relate to the cultures of XML's designers and XML language implementors, and a failure on the part of the digital library community to grapple with the sociotechnical nature of XML and its implementation. Possible strategies for addressing these issues of interoperability might include reduction of the flexibility afforded by specific XML-based markup languages used by the digital library community, and an increased focus on standardizing translations between various communities of practice use of such markup languages.