Abstract. Software requirements engineering is an essential activity for the successful development of information systems. Requirements can be specified using different techniques, but taking into account the different stakeholders that have to deal with requirements information, a successful approach has to balance the need to use a widely understood requirements notation, with the importance of eliminating the frequent ambiguity and imprecision, by adding rigor to the specification. In this paper we present the metamodel of a controlled natural language for interactive systems requirements specification, called ProjectIT-RSL, based on the identification of the linguistic patterns that are most frequently used in requirements documents written in natural language. Keywords. Requirements; Requirements Specification Languages; Controlled Natural Languages