In the last decades the scarcity of fossil fuels around the world has promoted the quest of new processes to produce power from alternative energy sources. The conventional Rankine cycle with water as working fluid is a good choice to convert heat into electricity, although it is inefficient for temperatures below 350 degrees C. Moreover, with the use of organic compounds mixtures as working fluid the Rankine cycle (called Organic Rankine Cycle) improves the cycle efficiency and the economy of the process for energy recovery from low temperature sources. In this work we propose a set of organic compounds to simultaneously obtain both optimum values of compositions of the mixture (used as working fluid) and optimal operating conditions of the cycle for a given low temperature source. As design goals we set the minimization of total annual cost and the maximization of thermal efficiency. Because these two goals are in conflict, the design problem was posed as a nonlinear multiobjective optimization problem with thermodynamic efficiency and total annual cost of the cycle as trade-off objective functions. In most cases binary mixtures are chosen for low cost and power ranges. On the other hand, as cycle efficiency improves, leading to expensive cycles, pure working fluids are preferred.