In this paper, we describe the ASP system, a testbed based on Web Services for coupled multi-physics simulations. The system is organized as a collection of geographically-distributed software components in which each component provides a Web Service and uses standard SOAP-based Web Service protocols to interact with other components. There are a number of advantages to organizing a system in this way, which we discuss. We have analyzed the performance of our system for a typical application and for a number of problem sizes, and have found that the overhead for using SOAP-based Web Services is small and tends to decrease as the problem size increases. Our results suggest that potential performance bottlenecks identified in the literature may not be major issues in practice, and that a standards-compliant implementation like ours can delivery excellent scalable performance even on coupled problems, provided Web Services are used judiciously.