The oxygen partial pressure in a furnace can be controlled precisely even at lower temperature (550°–900 °C), with a CO2–CO mixture preheated on an iron‐containing alumina catalyst for establishing chemical equilibrium in the gas phase. It was found that a CO2–CO system is more practical than a CO2–H2system because the optimum temperature of the catalyst is nearly equal to the main furnace temperature and does not depend on the mixing ratio of the gases.