Web services composition is a keystone towards the development of interoperable systems. However, desp ite several efforts for explicit semantic description, Web services still face data-heterogeneity chal lenges. Correct interpretation of data exchanged between composed Web services is hampered by differen t implicit modeling assumptions and representations. In this paper, we demonstrate the importance of c ontext to facilitate data exchange, and describe a context representation model for Web services. Then , we present a context-based mediation architecture that helps performing meaningful composition.