The practical development of sustainable energy sources is presently one of the key research topics due to its strong social and economic impact. Among the sources of sustainable energy, the use of low-temperature energy reservoirs has attracted interest. Normally, because of their low energy content, those processing streams are considered as non-profitable, especially when water is used as the working fluid in the thermodynamic cycles used for energy recovery from low-temperature energy sources. However, the efficiency of energy recovery can be increased using working fluids featuring low-temperature boiling points under the proper processing conditions. In this work we propose the optimal molecular design of a new family of organic fluids whose aim is to increase energy recovery from low-temperature energy sources. The design problem is cast as a mixed-integer non-linear programming problem, where binary variables are used to define the molecular structure of the working fluids and continuous variables permit the computation of physical and thermodynamic properties. The results indicate that optimal molecular design techniques permit the design of new organic compounds which increase energy recovery. Moreover, the toxicity of the new working fluids was reduced in comparison to other organic fluids used for the same purpose. (C) 2014 Published by Elsevier Ltd.