The Service-Oriented Architecture based on the Web service technology emerged as a conse- quence of the evolution of distributed computing. One of the key ideas of this technology is the ability to create service compositions by combining and interacting with pre-exisiting services. A service is implemented, described[1], and published by a service provider in a UDDI[2] registry. The service composition is referred to an executable process that interacts with other services ac- complishing its functional goal. Orchestration and Choreography[3] are the processes that allow to schedule the defined services compositions and messages exchanges. There is a wide range of industrial standardization efforts towards providing specification languages for the Web service composition. Among them BPEL (Business Process Execution Language[4]) is the most known and used orchestration language. Our work addresses the composition expressed by the orchestration and its support language BPEL. BPEL allows the designer to represent service compositions by var- ious behavioral properties like services interactions (message exchanges), control flow constraints (sequence, iteration, conditional) or data flow constraints (exchange, modification, evaluation of data expressions). Our work focuses on the formal verification of the composition of web services. We study the verification and validation of behavioral requirements through, the properties that a services com- position shall satisfy in order to achieve its functional goal. These requirements include deadlock freeness, correct manipulation and transformation of data, obeying to rules and to constraints on interactions ordering and termination. This verification is not supported by BPEL like languages although there exists several operational orchestration tools like Orchestra[5] that encode and in- terpret this language. We propose to address the problem of services composition verification using proof and refine- ment based techniques with the event B method. Our approach consists in extracting an event B model from service compositions written in BPEL. Thereafter, the obtained model is enriched with the relevant properties in the INVARIANTS and THEOREMS clauses and events guards. Two development scenarios have been studied.