The paper represents current achievements of an ongoing research that aims to develop a formal approach supporting an automatic selection of a Web service sought by a requestor. The approach is based on the matching the requestor's requirements for a "useful" service against the service description offered by the provider. We focus on the checking behavioral compatibility between operation contracts specifying pre-conditions and effects of required and provided operations. Graph transformation rules with positive application conditions are proposed as a visual formal notation for contracts. The desired dependence between requestor and provider contracts is determined by the semantic compatibility relation and syntactic matching procedure that is sound w.r.t. this relation.