Event notification plays an important role in the orchestration of distributed systems. Web services-based event notification is an important step to achieve an interoperable messaging system on the Internet scale. However, different incompatible specifications have been proposed to specify the Web services interfaces for Web services-based event notification systems. The WS-Eventing specification and the WS-Notification specifications are two major initiatives. WSMessenger is our implementation of both specifications. A mediation approach is used to reconcile the differences between these two specifications. In this paper, we propose a "Normalization-Processing-Customization" (NPC) model for mediation among competing Web services and show how this model is used in WS-Messenger to reconcile the incompatibility between these two specifications. The NPC model is flexible and scalable. The mediation implementation in WS-Messenger is efficient with a very small overhead.