The Web Service Description Language (WSDL) provides means to describe functional aspects of a service in a Service Oriented Architecture (SOA) based on Web Service technology. In contrast to its predecessor (WSDL 1.1), WSDL 2.0 does not define a fixed set of operation types but provides for a generic mechanism to define an operation by means of message exchange patterns (MEPs). In this paper we identify new MEPs and extend the template used to define MEPs to allow expressing more complex patterns. Furthermore, we compare the expressivity of MEPs in general with other work and formalisms in the field of service interaction. We give a refined definition of MEPs based on a detailed discussion and discuss how WSDL and the MEPs in particular can be combined with the choreography approach.