A compressed gas mixture, for example air, is passed through one of beds 12 and 14 of adsorbent that selectively adsorbs oxygen from the air. Unadsorbed nitrogen-enriched gas flows into a buffer vessel 28, from which product gas is withdrawn through pipeline 30. While one of the beds 12 and 14 is adsorbing oxygen for air the other is being regenerated, and during this period the communication between the beds is denied. At the end of a chosen period of time the beds are placed in communication with one another by opening valves 54 and 56. Then the bed that was previously being regenerated is placed in communication with the source of compressed air and the other bed regenerated. The period of intercommunication between the beds lasts for as long as it takes the volume of product gas as measured by an integrating flow meter 31 from the start of the preceding adsorption step to reach a chosen value. The apparatus is able to supply nitrogen of a chosen purity at a variable flow rate.