This work presents detailed simulations of Organic Rankine Cycle processes for energy conversion of low-enthalpy geothermal resources. The working fluids considered in this analysis are zeotropic mixtures. Second law efficiency of subcritical cycles is calculated for isobutane/isopentane and R227ea/R245fa depending on mixture composition, heat source temperature and temperature difference of cooling water. The use of mixtures as working fluids leads to an efficiency increase compared to pure fluids, due to a glide match of temperature profiles in the condenser and evaporator. For heat source temperatures below 120 degrees C the raise is up to 15%. In case of pure fluids a significant efficiency increase occurs, when the maximum pressure is reached and the pinch point shifts to the inlet of the preheater. This effect appears for R227ea at 120 degrees C and for isobutane at 170 degrees C. Using mixtures this behavior can be adjusted to higher temperatures by adding a less volatile component. (C) 2011 Elsevier Ltd. All rights reserved.