One of the challenging problems that Web service technology faces is the ability to effectively discover services based on their capabilities. We present an approach to tackle this problem in the context of DAML-S ontologies of services. The proposed approach enables to select the combinations of Web services that best match a given request Q and effectively computes the extra information with respect to Q (e.g., the information required by a service request but not provided by any existing service). We study the reasoning problem associated with such a matching process and propose an algorithm derived from hypergraphs theory.