This paper describes an approach to Enterprise Application Integration (EAI) using extensible Web services. The approach is demonstrated by building a real-world application for EAI in the financial services domain. Business drivers for and approaches to EAI are presented first. The manifestation of Web services in general and their role in EAI are discussed next. Financial services domain characteristics are presented. Business drivers that entail a strong need for functional extensibility in the financial services domain are described. Our proposed architecture for EAI which addresses functional extensibility is described. This architecture is based on the notion of extensible Web Services. We then present our implementation of the architecture and practical challenges encountered in EAI. A brief discussion of how our work relates to the current research in Service- Oriented Computing (SOC) and Semantic Web concludes the paper.