Abstract: A dual temperature isotope exchange process of the type having a frist stage comprising a hot and cold tower with liquid and gas passing in countercurrent exchange relationship therein, the gas being recycled from the top of the cold tower to the bottom of the hot tower via a humidifier section through which the effluent liquid from the bottom fo the hot tower also passes, a dehumidifier between hot and cold towers through which both gas and liquid streams pass, wherein a third-phase stream acting as a heat transfer medium is passed through the dehumidifier, hot tower, humidifier and wherein said stream is recycled from the bottom of the humidifier to the top of the dehumidifier or alternatively to the top of the cold tower via fluid condition and temperature treatment apparatus. The heat transfer fluid is preferably a liquid and must be such as not to interfere or inter-react excessively with the gas-liquid exchange process.