La2CuO4 is an active catalyst for the reduction of NO by CO. Under reaction conditions, the catalyst exhibits an activation which results in a lowering of the light-off temperature by 80 degrees C. XRD, TEM and EDX analysis carried out after the catalytic test indicate that the mixed oxide has been reduced to form a La2O3, Cu binary system. It seems that metallic copper species are the most active sites in the CO + NO reaction.