An optimisation model for the composition of mixed working fluids for (Organic Rankine Cycles) ORCs and Kalina cycles has been developed. The temperatures investigated were 100 degrees C-250 degrees C for the heat source and 30 degrees C for the heat sink. The optimisation method of the composition was carried out with Simulated Annealing technique with the objective function of maximising the thermal efficiency of the cycle, based on 1 MW of heat source. The results show that the pure component organic fluids are more energy-efficient than mixed organic fluids. The selection of organic working fluids was studied for achieving maximum cycle efficiency at a given operating temperature. The composition of the Kalina cycle was also optimised and it was found that for a maximum temperature of 250 degrees C, the minimum ammonia concentration in the ammonia-water mixture was 73.8% mol fraction. A novel consideration of employing alcohol-water mixtures in the cycle was also investigated and the most efficient mixture at 250 degrees C was methanol and water mixture when compared to the Kalina cycle and steam Rankine Cycle. The study showed overall that the optimal choice of working fluids for a particular type of cycle would depend on the operating temperature and pressure. The developed design method can be useful for engineers to evaluate the performance of the cycles considering a wide range of working fluids, and determines the optimal choice of working fluids and operating conditions of the cycle. (C) 2013 Elsevier Ltd. All rights reserved.