A process for the separation of air in a cryogenic distillation apparatus comprising a distillation column and in which air is separated to produce a fraction rich in oxygen and a fraction rich in nitrogen, comprising maintaining the purity of said oxygen product substantially constant during variations of demand for a said oxygen product or of the flow rate or of the pressure of the supply air by controlling the relative liquid to vapor interstage transfer rate within said distillation column in response to said variations of demand for a said oxygen product or of the flow rate or of the pressure of said supplied air, by increasing the relative liquid to vapor interstage transfer rate in said distillation column when the demand for said oxygen product or the flow rate of the supplied air increases, and decreasing the relative liquid to vapor interstage transfer rate in said distillation column when the demand for said oxygen product or the flow rate of the supplied air decreases, by withdrawing liquid rich in nitrogen from the distillation apparatus, and storing said liquid when the demand for said oxygen product or the flow rate of the supplied air decreases, and introducing at least a portion of said stored liquid rich in nitrogen into the distillation apparatus when the demand for said oxygen product or the flow rate of the supplied air increases.