Web services-based event notification is an emerging technology that combines the asynchronous communication feature of event notification mechanisms and the interoperability feature of Web services technologies. Web services-based event notification systems are important components for service-oriented Grid computing. WS-Eventing and WS-Notification are two major competing specifications for these systems. This paper is a comparative study of these specifications. The focuses of this research are on identifying the similarities and differences between these two specifications and identifying their evolutionary path from previous specifications. We found that competing Web services specifications take ideas and concepts from each other during the development progress, which is good for the maturity of the Web services-based event notification technology. We also identified several major changes from previous event notification systems to Web services-based event notification systems. In the end, we will present our WSMessenger project that supports both WS-Eventing and WSNotification specifications and provides mediation between them.