The effect of adding 330-4930 ppm hydrogen to a reaction mixture of NO and CO (2000 ppm each) over platinum and rhodium catalysts has been investigated at temperatures around 200-250 degrees C. Hydrogen causes large increases in the conversion of NO and, surprisingly, also of CO. Oxygen atoms from the additional NO converted are eventually combined with CO to give CO2 rather than react with hydrogen to form water. This reaction is described by CO + NO + 3/2H(2)-->CO2 + NH3 and accounts for 50-100% of the CO2 formed with Pt/Al2O3 and 20-50% with Rh/Al2O3. With the latter catalyst a substantial amount of NO converted produces nitrous oxide. Comparison with a known study of unsupported noble metals suggests that isocyanic acid (HNCO) might be an important intermediate in a reaction system with NO, CO and H-2 present.