The integration of GRID and MAS (Multi-Agents Systems) is an active research topic We have recently proposed the Agent-Grid Integration Language, to describe a service-based integration of GRID and MAS models However, the complexity of the mutual integration aspects leads us to define a rigorous way to formalize the key concepts, their relations and the integration rules by means of an ontology With this ontology, we can describe the elements and their composition that occur in various service exchange scenarios with agent on the Grid The ontology could be used both to model the behaviour of GRID-MAS integrated systems and to check the consistency of these systems and their instances A concrete scenario is illustrated.