Abstract: The concentration of one or more gaseous components of a mixture of gases is determined using a metallic oxide semiconductor the electrical resistance of which changes in the presence of reducing gases. The speed of reaction, relative to the speed of sorption and desorption, of different gases with the semiconductor varies at different temperatures. Since the speed of reaction of a hydrocarbon is substantially lower at certain temperatures than the speed of reaction of carbon monoxide, carbon monoxide can be indicated at those temperatures while, the variation in resistance determined by hydrocarbons is negligibly small.