We have evaluated the merits of organic-fluid mixtures as working media for Rankine power cycles. Non-isothermal phase change both at high and low temperature represents the main advantage with respect to pure fluids. StanMix, a computer code using the Wong and Sandler (WS) mixing rules, integrated into a commercial package, is employed for cycle analysis and optimisation. Heat recovery and geothermal applications using mixtures of siloxanes and hydrocarbons, respectively, are illustrated. We demonstrated that optimal selection of working-fluid composition is a powerful tool for an efficient ORC design.