Composite applications leveraging the functionalities offered by Web services are today the underpinnings of enterprise computing. However, current Web services composition systems make only use of functional requirements in the selection process of component Web services, while the transactional consistency and quality of service (QoS) are crucial parameters of most business applications. In this paper, we propose a novel approach for Web services selection and composition based on transactional requirements and QoS constraints. The transactional requirements are defined by the engineers using the acceptable termination states concept. We give a formalization of our approach in Event-B. We incrementally combine the modeling and verification activities related to formal development process supported by Event-B method. The verification activity is based on the proof and model-checking