Along with the emergence of ubiquitous computing there is a need for building reliable and secure applications that provide access to information in a continuous way. In such a context, access to resources and applications must be done in a flexible way through services that come up as a new paradigm for programming and organizing operations. In this paper we present an approach1 for building reliable mobile applications based on services oriented paradigm and the use of a contract based coordination model. Thanks to contracts, it is possible to associate a personalized behavior to a flow describing the logic of a mobile services based application. Contracts ensure, for example, transactional properties at execution time in the presence of exceptions and make applications aware of their execution context (QoS).