SOA (Service Oriented Architecture) defines a new Web services cooperation paradigm in order to develop distributed applications using reusable services. The handling of such collaboration has different problems that lead to many research efforts. In this paper, we address the problem of Web service composition. Indeed, various heterogeneities can arise during the composition. The resolution of these heterogeneities, called mediation, is needed to achieve a service composition. In this paper, we propose a sound approach to formalize Web services composition mediation with the ADL (Architecture Description Language) ACME. To do so, we first model the meta model of composite service manager and mediation. Then we specify semi formal properties associated with this meta model using OCL (Object Constraint Language). Afterwards, we formalize the mediation protocol using Armani, which provides a powerful predicate language in order to ensure service execution reliability