This paper presents a robust method for assessing the efficiency of organic Rankine cycle (ORC) plants based on the molecular structures of the working fluids employed. The developed methodology uses molecular group contribution methods and does not require equations of state or extensive experimental data. The maximum utilization efficiency eta u* of an ORC plant was correlated with two thermodynamic properties of the working fluid, namely, its critical temperature T-c and reduced ideal gas heat capacity C-p(0)/ R. The developed correlations predict rhi* with an average error of 0.9-1.5 percentage points. The optimum ORC heat source temperature T-hs* can be predicted with an average error of 3.5 degrees C to 6.6 degrees C. The developed methodology was validated using a numerical model of an optimized ORC. It was then used to estimate eta(u)* and T-hs* of 92 working fluids with low global warming potentials (GWP(100) < 150) and low flammability values (LFL > 0.1 kg/m(3)). Lastly, best candidate next-generation, low-GWP working fluids were selected for a more detailed examination. (C) 2017 Elsevier Ltd. All rights reserved.