Web services are becoming the basis for electronic commerce of all forms. The number of services being provided is increasing but different service providers use different ontologies for services’ descriptions. This has made it difficult for service discovery agents to compare and locate the desired services. Inputs and outputs are important pieces of information that can be used when searching for the needed services. Therefore, in this paper, to facilitate users or software agents for discovering Web services in multi-ontology environments, we propose an approach to determine the semantic similarity of services’ inputs/outputs that are described by different ontologies.