Abstract. This paper aims at defining the semantics of Agent Communication Languages (ACLs) in terms of changes in the social relationships between agents, represented in terms of social commitments. We take commitment to be a primitive concept underlying the social dimension of multiagent systems, and define a basic artificial institution that provides agents with the means to affect the commitment network that binds them to each other. Two different approaches are adopted for the presentation of our proposal: a logical formalization and an operational specification.