Abstract. In this paper we propose to regard an Agent Communication Language (ACL) as a set of conventions to act on a fragment of institutional reality, defined in the context of an artificial institution. Within such an approach, we first reformulate a previously proposed commitment-based semantics for ACLs. In particular we show that all commonly used types of communicative acts can be defined in terms of a single basic type, namely declarations, within an artificial institution that we call Basic Institution. We then go on defining special institutions, that augment the Basic Institution by adding ontological and normative elements. Finally, as an example of a special institution we give a partial definition of the institution of English Auctions.