This paper presents semantic approach for a solution of the problem of interoperability for virtual worlds. The semantic of virtual worlds and virtual entities is described using the Influence/Reaction model issued of multi-agent systems. The main feature of this model is to clearly separate the description of an action (performed by virtual entities) and the consequences of the action. We demonstrate how this separation is highly favourable to design virtual worlds with interoperabilty capability. We use and extend the Influence/Reaction model to quantify the semantic compatibility level between virtual worlds and then to compute the resulting behavior of virtual entities involved in an interoperability session. Nevertheless, the Influence/Reaction model can work properly for interoperabilty purpose only if the symbols (names) used to describe the virtual entities are identical in each virtual world. This constraint is relaxed by using the ontology concept.