The organic Rankine cycle (ORC) has been widely used to convert low-grade (<300 degrees C) thermal energy to electricity. Use of zeotropic mixtures as the working fluids improves the thermodynamic performance of ORC systems due to better matches of the temperature profiles of the working fluid and the heat source/sink. This paper presents a method to determine the optimal ORC condensation pressure when using a zeotropic mixture. This study also investigates the effects of the condensation temperature glide of the zeotropic mixture on the ORC thermodynamic performance. Geothermal water and biomass are used as the heat sources. Zeotropic mixtures of butane/pentane (R600/R601), butane/isopentane (R600/R601 a), isobutane/pentane (R600a/R601) and isobutane/isopentane (R600a/R601a) were selected as the working fluids for the geothermal ORC with octane/decane, nonane/decane and octamethyltrisiloxane/decamethyltetrasiloxane (MDM/MD2M) selected as working fluids for the cogenerative ORC driven by the biomass energy. Two optimal working fluid mole fractions maximize the cycle efficiency, exergy efficiency and net power output for cooling water temperature increases less than the maximum condensation temperature glide, while the highest net power output appears at the higher mole fraction of the more volatile component for the geothermal ORC when the condensation temperature glide of the working fluid mixture matches the cooling water temperature increase. Higher condensation temperature glides result in large thermal loss to the heat sink and exergy destruction in the condenser. There is only one optimal working fluid mole fraction that maximizes the thermal efficiency, exergy efficiency and net power output when the cooling water temperature increase is greater than the condensation temperature glide. (C) 2013 Elsevier Ltd. All rights reserved.