In this paper, we are motivated by the problem of automatically and dynamically selecting and composing services for the satisfaction of user requirements. We propose an approach dealing with requirements freely expressed by the user, and in which agents perform service composition through unplanned interactions. Our architecture is based on agents that offer semantic web services and that are capable of reasoning about their services’ functionalities. We propose to provide such agents with an interaction protocol that allows them, through dialogues, to select and compose appropriate services’ functionalities in order to fulfill a complex set of requirements specified by a user.