With the rising popularity of Web services, both academia and industry have invested considerably in Web service description standards, discovery, and composition techniques. The standards-based approach utilized by Web services has supported interoperability at the syntax level. However, issues of structural and semantic heterogeneity between messages exchanged by Web services are far more complex and crucial to interoperability. It is for these reasons that we recognize the value that schema/data mappings bring to Web service descriptions. In this article, we examine challenges to interoperability; classify the types of heterogeneities that can occur between interacting services, and present a possible solution for data interoperability using the mapping support provided by WSDL-S, a key driver behind SAWSDL. We present a data mediation architecture using the extensibility features of WSDL and the popular SOAP engine, Axis 2.