Web services exposing run-time behaviour that deviates from their behavioural specifications represent a major threat to the sustainability of a service-oriented ecosystem. It is therefore critical to verify the behavioural conformance of services during run-time. This paper discusses a novel approach for run-time verification of Web services. It proposes the utilisation of Stream X-machines for constructing formal behavioural specifications of Web services which can be exploited for verifying that a service’s run-time behaviour does not deviate from what is defined in the specification. Our approach allows for checking both the control flow of a Web service and the values of the data in the generated responses. The paper also proposes a classification of Web services and discusses how different types of services can be verified at run-time. Finally, it presents a run-time monitoring and verification architecture and discusses how it can be integrated into different types of service-oriented infrastructures.