A new ultrathin-backed semipermeable membrane has been developed which shows considerable promise as a gas separator for engine bleed air to provide nitrogen-rich air for aircraft fuel tank inerting. The membrane is a silicone, polycarbonate copolymer of 1500 Å effective thickness, deposited on a reinforced porous backing. The selective removal of oxygen provides oxygen concentrations of less than 9% in the inerting gas. Small-scale testing demonstrated that the backed membranes are suitable in the aircraft environment. A system using such membranes avoids the logistic and service requirements of tanked liquid nitrogen.