Abstract: A process and installation for the separation of air in a cryogenic distillation apparatus (24) comprising a distillation column (30, 42) and in which the supplied air is separated to produce a fraction rich in oxygen and a fraction rich in nitrogen as products. The purities of these products are maintained substantially constant during variations of demand of either product or of the flow rate or of the pressure of the supplied air by introducing an excess of liquid rich in nitrogen into the distillation apparatus when the demand for the product or the flow rate of the supplied air increases, and by withdrawing an excess of liquid rich in nitrogen from the distillation apparatus, and storing this liquid, when the demand for the product or the flow rate of the supplied air decreases.