The concentration of a constituent gas in a product gas delivered by a molecular sieve type gas separation system 10 is controlled by a flueric partial pressure sensor 15 connected to a spill-valve 16 which is conduitly connected to a product gas delivery duct 13. The flueric partial pressure sensor 15 compares a reference bleed of supply gas with a sample bleed of product gas and outputs signals to control the spill-valve in spilling product gas from the delivery duct 13 so that the rate of flow of gas through adsorption beds 11 of the system 10 is controlled to provide product gas having a required concentration of the constituent gas.