An ontology defines a common vocabulary that facilitates the sharing of information in a domain. It includes machine-interpretable definitions of basic concepts in the domain and their relations. In the Grid environment (GE) where so many different implementations are available, the need for semantic matchingbased on a defined ontology becomes increasingly important. Especially for service discovery (SD), an ontology description is very useful in supporting a customised SD process. This paper proposes a SD matchmaking framework for GEs based on a well-definedontology. The matching of service requests is discussed and a matchmaking mechanism is described in detail. The proposed framework allows for close and customised SD matches, that current service discovery systems (SDSs) cannot provide as their SD process is based on type matching only.