A dynamic apparatus was developed for producing mixtures of carbon monoxide with air which is reported to make it possible to obtain gas mixtures over a wide range concentrations. The difference between this apparatus and other known ones lies in the use of an original dosing device for pure monoxide, which consists of a flexible polyethylene bag, placed in a metallic can filled with carbon monoxide. Air, entering the bag under pressure, uniformly expands, and, as the bag fills, carbon monoxide is displaced from the metallic can. The dosing of carbon monoxide thus proceeds uniformly, over a long time period.