Abstract: Ambient air, during an on-stream period, is passed through a first adsorbent bed removing moisture and carbon dioxide and then through a second bed of adsorbent selective for retention of nitrogen, the oxygen-rich effluent being collected in an expandable receiver. Desorption of both beds is effected by reducing the pressure at the inlet end of the first bed causing desorbed nitrogen from the second bed to pass through the first bed, desorbing moisture and CO.sub.2 therefrom. Further removal of moisture and CO.sub.2 from the first bed is then effected by lowering the pressure of that bed further, while the second bed is restored to its on-stream pressure by admission of enriched oxygen thereto from the expandable receiver.