Determining the QoS (quality of service) of composite Web services is of crucial importance for both service providers and consumers. However, service descriptions of constituent services are often heterogeneous which makes the aggregation of multiple single-service QoS difficult. In this paper, we address the heterogeneity problem from the perspective of knowledge engineering. We propose a QoS aggregation ontology; i.e., a formal parameter classification which can be used to determine the right aggregation for annotated QoS parameters based on workflow patterns.