Suppression of NH3 formation over NOx catalysts has been the subject of several reports in recent years. However, there are very few reports relating to the effects of modifications of both feed gas conditions and catalyst usage. Therefore, NH3 formation was investigated as a function of such feed gas boundary conditions as CO concentration, CO/O2 ratio, space velocity, linear velocity and catalyst bed temperature. Both fresh and aged NOx catalysts were used. Longitudinal patterns of NH3 formation in monolithic NOx catalyst were also determined. Based on these results, several ideas were tested to reduce NH3 formation in the NOx catalyst through control of feed H2, O2, CO and HC concentrations. Finally, the possibility of extending the optimum A/F range of a NOx catalyst toward leaner A/F ratios by the introduction of methane ahead of the NOx catalyst is discussed.Although these studies increased our knowledge of NOx catalysis and use of NOx catalysts, many engineering problems still remain to be solved before NOx catalysts are used in production vehicles.