The determination of the most suitable operative fluid for the various types of Organic Rankine Cycle expansion machines has been the object of an abundant literature. In this study an analysis was carried out in a rotary volumetric expander based on the Wankel concept, which was never studied before for this application. This paper is focused in assessing the proper coupling between the temperature at which the heat is available, and hence the saturation temperature at the inlet, and the working fluid that is to be used in an expander of given displacement to get the desired mechanical power. This analysis was carried out by means of a numerical model of the expander that was validated by comparison with the data measured at the dyno on a prototype that was built at the University of Pisa and operated with compressed air. The comparison was carried out in terms of indicated pressure, delivered torque and working fluid mass flow rate. The operative maps of this expander were evaluated by considering the working conditions that give an isentropic efficiency higher than a fixed value. By this way, it was possible to evaluate the relationship between the used working fluid, the inlet temperature and the delivered mechanical power. (C) 2013 Elsevier Ltd. All rights reserved.