Web services are designed for composition and use by third parties through dynamic discovery. As such, the issue of interoperability between services is of great importance to ensure that the services can work together towards the overall application goals. In particular, the interaction protocols of a service need to be implemented and used properly so that the service composition can conduct itself in an orderly fashion. In our previous work, we have proposed a lightweight, pattern/constraint-based approach to specifying service interaction protocols. It has been incorporated into OWL-S for service developers to describe service interaction constraints. In this paper, we present a framework for monitoring the run-time interaction behaviour of Web services and validating the behaviour against their pre-defined interaction constraints. The framework involves interception of service interactions/messages, representation of interaction constraints using finite state automata, and conformance checking of service interactions against interaction constraints. As such, the framework provides a useful tool for validating the implementation and use of services regarding their interaction behaviour.