Organic Rankine cycles (ORCs) are preferred to convert low temperature geothermal energy (<150 degrees C) to electricity. The working fluid selection and system parameters optimization are the main approaches to improve geothermal ORC systems performance. Zeotropic mixtures are showing promise as ORC working fluids due to the better match between the working fluid and the heat source/sink temperatures. This study optimizes the cooling water temperature rise as well as the evaporation and condensation pressures of isobutane/isopentane (R600a/R601a) mixtures for various mole fractions to maximize the net power output of a geothermal ORC for geothermal water temperatures of 110 degrees C, 130 degrees C and 150 degrees C and reinjection temperatures not less than 70 degrees C. Two mole fractions maximize the turbine power generation, while the maximum net power output occurs for R600a mole fractions from 0.7 to 0.9 due to the variation of the parasitic power consumed by the working fluid feed pump and the cooling water circulating pump. A geothermal ORC using R600a/R601 can generate 11%, 7% and 4% more power than an ORC using pure R600a for geothermal water inlet temperatures of 110 degrees C, 130 degrees C and 150 degrees C, respectively. Both the evaporator and condenser area per unit power output using R600a/R601a are higher than that using pure R600a or R601a due to the reduction in the heat transfer coefficient and the temperature difference between the mixture working fluid and the heat source/sink. The total heat transfer area per unit power output increases to a maximum at an R600a mole fraction of 0.5 and then decreases with increasing R600a mole fraction. The total heat transfer area can be reduced for the same power output with higher geothermal source temperatures. The turbine using R600a/R601a with higher R600a mole fractions is nearly the same size as a turbine using pure R600a. (C) 2015 Elsevier Ltd. All rights reserved.