In this paper, a graph formalism is proposed to describe the semantics of documents in a multilingual context. This formalism called semantic graph is an extension of the conceptual graphs of Sowa. Semantic graphs are based on a semantic thesaurus which stores two kind of knowledge: domain knowledge and lexical knowledge. The goal of semantic thesaurus is to share a common semantic for different languages. Thus, semantic graphs enable to develop several useful functionalities for a multilingual system. For example, users can compose their queries using a thesaurus written in their language. Resulting documents can be adapted to the user language etc…. All these functionalities are implemented in our multilingual documentary system, called SyDoM.