Results of thermodynamic studies of geothermal steam power systems incorporating fossil-fired superheaters are reported. One- and two-stage systems are covered. Realistic assumptions have been included to account for losses that may be incurred in actual plants. The systems are evaluated on the basis of a number of appropriate figures of merit. It is found that fossil superheat hybrid geothermal power plants offer a thermodynamic advantage over individual fossil and geothermal plants for a wide range of operating conditions, and deserve consideration whenever fossil and geothermal energy resources are found in reasonable proximity