Abstract The influence of small amounts of natural gas, C 2H 6 and CO on the homogeneous gas phase reduction of NO x by NH 3 was investigated in air-rich flue gas from an oil-fired boiler (1 MJ s −1) at temperatures from 1100 K to 1350 K. It was found that the addition of natural gas, C 2H 6 and CO to the NH 3 injected into the flue gas decreased the optimum reaction temperature by 150 to 200 K. The maximum removal efficiency of NO x (75%) was not strongly affected by the additives. The effect of the additives on the NO x reduction involves three reactions. 1. (1) The reduction of NO x by NH 3 is accelerated by the additives (natural gas, C 2H 6 and CO). 2. (2) The oxidation of NH 3 to NO x by O 2 present in the flue gas is also accelerated by the additives. 3. (3) The additives are oxidized to CO 2 and H 2O by oxygen. The oxidation of CO, which is also an intermediate step during the oxidation of hydrocarbons, is inhibited by unreacted NH 3.