Services are oered in an execution context that is determined by how a provider provisions the service and how the user consumes it. The need for more exibility requires the provisioning and consumption aspects to be addressed at runtime. We propose an ontology-based context model providing a framework for service provisioning and consumption aspects and techniques for managing context constraints for Web service processes where dynamic context concerns can be monitored and validated at service process run-time. We discuss the contextualization of dynamically relevant aspects of Web service processes as our main goal, i.e. capture aspects in an extended context model. The technical contributions of this paper are a context model ontology for dynamic service contexts and an operator calculus for integrated and coherent context manipulation, composition and reasoning. The context model ontology formalizes dynamic aspects of Web services and facilitates reasoning. We present the context ontology in terms of four core dimensions - functional, QoS, domain and platform - which are internally interconnected.