Abstract: Aerobically decomposable liquors are oxygenated by continuously extracting a portion thereof from an aeration tank and saturating the liquor portion with oxygen within a separate receiver tank, thereafter returning the oxygenated liquor portion to the aeration tank. The continuously flowing liquor portion is cooled prior to its oxygenation so that its oxygen-absorbing capability is increased. Cooling is effected by extracting therefrom the heat required to gasify the initially liquified gas being introduced into the receiver tank as the oxygenating medium. Secondary cooling of the liquor portion before its oxygenation can be effected by extracting therefrom an additional amount of heat as required to gasify a second liquified gas, such as nitrogen.