Contributions in the field of Web services have identified that (a) finding matches between semantic descriptions of advertised and requested services and (b) non-functional characteristics - the Quality of Service (QoS) - are the most crucial criteria for composition of Web services. In this work a mechanism is introduced that determines the QoS of a Web service composition by aggregating the QoS dimensions of the individual services. This allows to verify whether a set of services selected for composition satisfies the QoS requirements for the whole composition. The aggregation performed builds upon abstract composition patterns, which represent basic structural elements of a composition, like sequence, loop, or parallel execution. This paper focusses on workflow management environments. We define composition patterns that are derived from Van der Aalst's et al. comprehensive collection of Workflow Patterns. The resulting aggregation schema supports the same structural elements as found in workflows. Furthermore, the aggregation of serveral QoS dimensions is discussed.