The mechanism of reduction of nitric oxide by hydrogen and carbon monoxide was studied on base metal catalysts under simulated automobile exhaust conditions. The catalysts were low copper-nickel alloys supported on an all-metal open-mesh substrate. Space velocity and concentrations of inlet and outlet NO, H2, CO, and O2 were used as study parameters. Dependence of ammonia formation on the hydrogen content in the gas was established. It was found that the dependence of ammonia formation on the oxygen content could be accounted for by the reaction of oxygen with the hydrogen produced by the water-gas reaction. In general, indications were obtained that under the conditions studied, the reduction of NO on base metal catalysts proceeds by a mechanism in which ammonia is an intermediate.