The feature interaction problem can be managed by the use of abstract models related by the refinement relationship. A service is incrementally built with respect to the requirement and the combination of services is defined as an instance of the refinement relationship. We use the B method and especially the event-based approach and we show how features and services can be safely combined to obtain a sound model of combined services. Two refinements are defined following directions of refinement and the refinement-as-composition principle is developed with the B-event-based approach called B system. Although service composition is non monotonic, B system provides a framework for analysing services and services composition.