A method for starting fuel cell power systems comprises heating a fuel cell stack to a predetermined operating temperature with a circulating heat-exchanger fluid, while feeding a pressurized inert gas into a pressure vessel and process gas chambers of the fuel cell stack to increase pressures in the process gas chambers and pressure vessel to a predetermined pressure, detecting a temperature of the stack and a pressure of the inert gas in the process gas chambers, and controlling a vapor pressure in the inert gas fed to said chambers in proportion to the detected temperature and pressure in the process gas chambers so that a volume of an electrolyte is approximately kept constant.