Abstract: An inflatable vehicle occupant restraint system comprises a source of inflation fluid (20) and an air bag reaction canister (24) having a vent opening (16) through which inflation fluid from the source of inflation fluid (20) may be vented away from the air bag canister (24). An air bag (12) is located in the air bag reaction canister (24) and is inflatable by inflation fluid from the source of inflation fluid (20). A valve member (30) is located on the air bag reaction canister (24). The valve member (30) is movable relative to the vent opening (16) in response to ambient temperature variations in the vicinity of the source of inflation fluid (20) to control venting of inflation fluid from the source of inflation fluid (20) through the vent opening (16). Inflation of the air bag (12) is thereby controlled in response to ambient temperature variations in the vicinity of the source of inflation fluid (20).