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.