This paper introduces a formal model for the description of interactive systems based on the interactor model of [15, 17]. Similarly to that model, it is intended to be used constructively for building specifications of interfaces as compositions of interactors. Changes are brought about to two aspects of the model: firstly, a modularised representation of control information is achieved which supports the independent description of the data transforming behaviour of the interactor and of the temporal constraints imposed on that behaviour. Secondly, distinct representations of ‘result’ and ‘display’ data handled by an interactor are related within a process algebraic framework, allowing the expression of usability related properties of interaction.