The air flow control valve inlet is fed by a pump and the outlet (38) introduces additional air flow into the catalyser during start-up when the engine is cold. The valve action is provided in valve seat (86) sealed off by flexible ring (110) that can open when a valve disc (122) is clear of the surface. The disc is mounted on a spindle coupled to a diaphragm that closes when the pump is stopped. USE - Supply to catalyser for i.c. engine of motor vehicle.