This paper presents a novel technique that significantly improves the quality of semantic Web service matching by (1) automatically generating ontologies based on Web service descriptions and (2) using these ontologies to guide the mapping between Web services. The experimental results indicate that with our unsupervised approach we can eliminate up to 70% of incorrect matches that are made by dictionarybased approaches. Categories and Subject Descriptors C.2.0 [Computer Systems Organization]: COMPUTERCOMMUNICATION NETWORKS--Data communications General Terms Algorithms, Standardization Keywords Semantics, Web services