This paper proposed a dimensionless term, the "Figure of Merit" (FOM), to investigate the thermal performance of a low temperature, organic Rankine cycle using six zeotropic mixtures (R245fa/R152a, R245fa/R227ea, R245fa/11236ea, R245ca/R152a, R245ca/R227ea and R245ca/R236ea) as working fluids. An empirical correlation was developed to estimate the cycle efficiency from the FOM for all working fluids at condensing temperatures of 25-40 degrees C and evaporating temperatures of 80-130 degrees C. The model results fit very well with both the experimental data and that from other researchers. (C) 2015 Elsevier Ltd. All rights reserved.