The reduction of NO by CO is performed over palladium supported on activated carbons and on a graphite of lower surface area. The presence of very low amounts of Pd gives active and stable catalysts towards the NO-CO reaction and the contribution of the surface carbon atoms to the process is negligible, the presence of CO lowering the consumption of the carbon support by NO. In the absence of any other reductant in the gas phase, NO is reduced by reaction with the carbon atoms. In the presence of an excess of O-2 (net-oxidizing NO-CO-O-2 mixtures), the consumption of the carbon supports by O-2 is important. The reactions of NO with the C atoms are also inhibited, NO reacting with CO but O-2 reacts with both reductants, CO and C.