The availability of many independent services on anopen network opens the opportunity of composing individualinstances to achieve complex functionality. Most often thereare several possible compositions to achieve the same highlevelfunctionality; the advantage of choosing one compositioninstead of another one may lie in the different quality ofthe composition, e.g., one might be cheaper, faster, or morereliable. In this paper, we focus on services described withXML documents and accessed via XML Protocols, knownas Web services, and enriched with semantic and Qualityof Service (QoS) annotations. We propose an algorithmthat, given a desired functionality, returns a composition ofservices from a repository with the optimal response time orthroughput. Services are composed taking into account anontology of operation names expressed in OWL. RuGQoS isthe related implementation.