Abstract: A process and apparatus provides breathing gas for occupants of pressurized cabins, e.g., cabins in aircraft, by passing a compressed air through adsorbers for increasing the relative oxygen concentration thereof. The same apparatus is simultaneously employed for providing an emergency breathing gas supply stock. In normal operation, the supply from the adsorber is selectively switched to partially flow into an emergency storage tank for storing enriched oxygen breathing gas therein, and then switched back into supplying a total flow of the normal breathing gas for the occupants of the pressurized cabin. In the event of a system shut-down, the thus provided emergency storage tank is switched on to supply the emergency breathing gas.