Abstract: The universal mixer device for mixing two gaseous fluids comprises an enclosure defining coaxial chambers, ducts opening out respectively into each of the chambers to feed them with two gases at medium pressure, and sonic nozzles each comprising a hollow converging-diverging body of revolution serving as seats for respective cone-shaped valve members. The valve member of one of the nozzles has an axial bore and defines the chambers at least in part. The other nozzle is disposed inside the axial bore. The hollow converging-diverging bodies of revolution occupy positions that are determined relative to the enclosure, and the valve members are mechanically linked to each other and are associated with a single actuator. The gases that have passed through the coaxial and geometrically similar nozzles flow into a common downstream chamber in which the gases mix with a predetermined mixing ratio determined by the dimensions of the nozzles.