General guidelines on how to choose the optimal working fluid based on the hot source temperature available are reported. Based on a systematic approach, 26 commonly used working fluids are investigated by optimisations at hot source temperatures in the range 50-280 degrees C at intervals of 5 K. The genetic optimisation algorithm is used to optimise net power output by an optimal combination of turbine inlet pressure and temperature, condenser pressure, hot fluid outlet temperature, and mixture composition for mixtures. The results suggest that the optimum working fluid in terms of maximum net power output has a critical temperature approximately 30-50 K above the hot source temperature. When two or more fluids with the same critical temperature are available, the ones with a positive slope of vapour saturation line are generally favoured. When mixtures are considered, the optimal mixture composition should be chosen so that the critical temperature of mixture is approximately 30-50 K below the hot source temperature and the temperature glide during condensing should approximate the temperature rise of the cold source. (C) 2015 Elsevier Ltd. All rights reserved.