Abstract: A method and device for control of the carbon level of a gas mixture reacting in a heat treatment furnace, which gas mixture arises by means of the introduction of a fuel containing hydrocarbon into the furnace chamber, the reaction products of which are not in a state of water-gas equilibrium and not in a state of methane gas equilibrium and which gas mixture has an excess of methane. The controlled condition is determined by a first measurement device from the portion of the gas component CO which is present in the furnace chamber as a first measured value, and by a second measurement device from the electrical voltage of an oxygen-ion-conducting solid body electrolyte as a second measured value and by a third measurement device from the furnace chamber temperature as the third measured value. A regulating member is controlled as a result to automatically change the quantity flow of the supply to the furnace chamber until correspondence of the measured carbon level with its desired value exists.