Enabling the Semantic Web requires solving the semantic heterogeneity problem, for which ontology matching methods have been proposed. These methods rely on similarity measures that are mainly focused on terminological, structural and extensional properties of the ontologies. Semantics rarely play a direct role on the ontology matching process, albeit some algorithms have been proposed. On the other hand, many ontology engineers choose representation languages that have an underlying formal logic, providing well-defined model-theoretic semantics. Since semantics are a key advantage of ontologies, we believe that semantics-based similarity measures are crucial. In this paper, we present a novel approach to semantic similarity. Key words: ontology matching, semantic similarity