Determining the QoS (quality of service) of composite Web services is of high importance for both service providers and service consumers. Heterogeneity of service descriptions, however, often hinders the aggregation of QoS parameters. We propose ontology-based QoS aggregation that integrates the semantics of QoS parameters and their aggregation into the overall aggregation process. The contribution is a QoS aggregation ontology and a QoS aggregation method that uses this ontology. We demonstrate the usefulness of our proposal for designers of composite services and assess its computational efficiency.