Communities gather Web services that provide a common functionality, acting as an intermediate layer between end users and Web services. On the one hand, they provide a single endpoint that handles user requests and transparently selects and invokes Web services, thus abstracting the selection task and leveraging the provided quality of service level. On the other hand, they maximize the visibility and use rate of Web services. However, data exchanges that take place between Web services and the community endpoint raise several issues, in particular due to semantic heterogeneities of data. Specific mediation mechanisms are required to adapt data operated by Web services to those of the community. Hence, mediation facilititates interoperability and reduces the level of difficulty for Web services to join and interact with communities. In this chapter, we propose a mediation approach that builds on (1) context-based semantic representation for Web services and the community; and (2) mediation mechanisms to resolve the semantic heterogeneities occuring during data exchanges. We validate our solution through some experiments as part of the WSMO framework over a test community and show the limitations of our approach. © 2009 Springer-Verlag Berlin Heidelberg.