Abstract: A molecular sieve type gas separation system capable of delivering oxygen-enriched air by increasing the oxygen content of air fed to the system has control means including a valve arrangement associated with each sieve bed for cyclically subjecting each bed to a charge/adsorption on-stream phase followed by a purge/desorption regeneration phase. During the regeneration phase of each sieve bed the gas pressure in the bed at least substantially equals the ambient atmospheric pressure or a pressure related thereto such as the ambient pressure of an outlet to which the oxygen-enriched air is being delivered and, for a predetermined range of outlet ambient pressure, the overall cycle time and the relative durations of the phases are fixed at values such that the oxygen content of air delivered by the system remains within physiologically acceptable limits for breathing.