A demand valve for gas mixing comprises two or more flow control valves that are connected to separate gas supplies. The demand valve includes a common actuation mechanism, such as a diaphragm arranged to displace levers, each of which, in turn, opens and closes proportionally in a substantially constant ratio a respective flow control valve. Inhalation flow drawn through an outlet causes displacement of the diaphragm and hence opening of the flow control valves. Gas from the separate supplies is therefore mixed in a main chamber to a substantially constant composition, regardless of the flow rate demanded, before being dispensed via the outlet.