The effect of using mixtures as working fluids in organic Rankine cycles (ORCs) is examined with a simulation model of the cycle that includes all elements of an actual installation. We consider several of the commonly used pure ORC fluids as potential components, discuss a mixture selection method, and suggest optimal concentrations. For heat sources at 150 degrees C and 250 degrees C, a potential increase of 16% and 6% in cycle efficiency is found. The electricity production at optimal thermal power recuperation can be increased by 20% for the low temperature heat source. (c) 2012 Elsevier Ltd. All rights reserved.