Abstract: An operating system for dual-sieve oxygen concentrators, as used by invalids, provides a method and apparatus for controlling the purging and release of product gas and the cycling period in response to the pressure build-up in each sieve column. The cycling period is thereby self-adjusting to changes in the capacity of the columns. Restricting flow of product gas and purge gas to the higher-pressure part of the adsorption period provides product gas of higher purity and reduces the loss of oxygen in the purge.