Program GEOTHM is a thermodynamic process program now under development for the LBL Geothermal Energy Program. To date, the program development has centered upon the modeling of working fluid properties, developing thermodynamic process models, and modeling the design performance of geothermal power plants. When the program is completed, it will be able to optimize a power plant or refrigeration plant for minimum cost power or refrigeration. Furthermore, operation of the thermodynamic cycles at off design conditions will be able to be simulated. Program GEOTHM is currently able to calculate several types of geothermal power cycles using a wide variety of working fluids.