Abstract: A method for the control of the operation of an installation for the separation of air by cryogenic distillation comprising a double distillation column wherein the top of a medium pressure column and the base of a low pressure column are thermally interconnected by a vaporizer-condenser supplied with liquid oxygen from a bath of liquid oxygen in the base of the low pressure column. Liquid oxygen from the bath is continuously tested to detect the content of nitrous oxide in the bath, and upon the detection of a fall in the nitrous oxide content of the bath when the installation is operating under equilibrium conditions, the flow of liquid oxygen to the vaporizer-condenser is increased to raise the detected nitrous oxide content of the bath. But when the detected nitrous oxide level falls, then either a liquid oxygen purge line is opened farther, or else the operating time or pressure of switching adsorbers in the air supply is changed.