Abstract: A breathing gas supply system for an aircraft includes a plurality of oxygen concentrating apparatus, each of which in use, is operable to supply oxygen enriched gas to a breathing gas supply. Each oxygen concentrating apparatus includes at least two active molecular sieve beds which are operable so that while one sieve bed is adsorbing non-oxygen gas from a pressurized gas supply, the or another bed is being purged of non-oxygen gas by subjecting the bed to lower pressure. Each oxygen concentrating apparatus includes an oxygen enriched gas flow control device which permits the flow of oxygen enriched gas produced by the oxygen concentrating apparatus to the breathing gas supply and permits a restricted flow of oxygen enriched gas from the breathing gas supply to the oxygen concentrating apparatus. Oxygen enriched gas produced by the adsorbing sieve bed flows direct to the bed being purged.