The paper deals with two methods of determining the efficiency of combined gas-and-steam power stations. For each of them examples of calculations have been quoted. These two methods permit to determine the efficiency of investigated systems as a func-tion of temperature at the inlet to the gas turbine and the compression ratio for the opti-mal steam part of the system. Calculations have been carried out concerning various structures of power stations with a single-pressure waste-heat boiler, a double-pressure waste-heat boiler with and without reheat and a triple-pressure waste-heat boiler and reheat.