Several web services compositions languages and standards are used to describe different applications available over the web. These languages are essentially syntactic ones, their descriptions remain informal and are based on graphical notations. They do not offer any guarantee that the described services achieve the goals they have been designed for. The objective of this paper is twofold. First, it focusses on the formal description, modelling and validation of web services compositions using the Event B method. Second, it suggest a refinement based method that encodes the BPEL models decompositions. Finally, we show that relevant properties formalized as Event B properties can be proved. A tool encoding this approach is also available.