Abstract: Gas mixing valve having separate inlets for two gases, a common outlet and a dispensing device located between said inlets and the outlet, and a reflux preventing valve, said dispensing device consisting of an adjustable slide block with a number of functionally parallel-connected gas channels, one end of which is connectable to one or the other inlet and the other end of which is closable under spring action by means of a valve flap common to the gas channels and arranged to open to different extents for different flows but equally for all individual gas channels.