Abstract: A power cycle operating with a maximum temperature above 300.degree. C. The power cycle operates with a working fluid comprising a mixture of water and another substance having lower volatility, greater molecular weight and tendency to superheat in isentropic expansion. Both substances are vaporized in a boiler, in part at variable temperature, and expanded in at least one turbomachine. After the first expansion, heat is yielded at constant pressure, wherein part of the least volatile substance condenses at variable temperature. In comparison with a steam cycle, this new cycle offers higher efficiences because it has the advantage of increasing the average temperature of heat absorption, without intermediate reheating and without condensation occurring in the turbine until very low exhaust pressures are reached, depending on the proportion of the mixture used. A secondary cycle of refrigerant fluid may be utilized with the power cycle of the present invention.