The organic Rankine cycle is one of the most promising techniques for production of mechanical power from low temperature sources such as solar heated fluids, geothermal brines and industrial waste heat. A series of cycle performance calculations has been carried out for a broad range of working fluids including Refrigerant 12, isobutane, isopentane, propone, and toluene as well as mixtures of isobutane and isopentane. For a source temperature range of 150°C to 170°C, current results show that the optimal cycle for some working fluids requires supercritical heat exchange while for others (the majority), heating is subcritical. Performance improvements available from exhaust fluid regeneration are shown to be more significant for an isobutane cycle (subcritical) than for a propane cycle (supercritical). For the foregoing cycles the amount of heat exchanger area per unit of output is compared; the isobutane cycle is found to require somewhat less area. The sensitivity of cycle output to changes in pump and turbine efficiencies is also illustrated.