The addition of hydrogen or water to a stoichiometric mixture of CO and NO over supported platinum and rhodium catalysts can significantly increase the conversion of both NO and CO. Theselectivity of NO reduction is also affected in a remarkable manner. With Pt/Al 2O 3 at 210°C, for example, addition of, ≈900 ppm H 2 increases the NO conversion from 6% to 48% and that of CO from 5% to 33%. The nitrogen atom of NO is converted quite selectively to NH 3 while the oxygen atom forms CO 2 rather than water. Addition of, ≈1400 ppm water also enhances both CO and NO conversion over Pt/Al 2O 3 but N 2 rather than NH 3 is the favoured product. Hydrogen and water also produce significant rate enhancements with Rh/Al 2O 3 and Rh/CeO 2/Al 2O 3 but the product distributions are somewhat different from those with Pt/ Al 2O 3. Possible reasons for the enhanced activities and the observed selectivities are discussed.