The extent to which isocyanic acid (HNCO) is formed during the reaction of NO/CO/H-2 mixtures over silica-supported Pt, Rh, and Pd has been investigated together with the subsequent hydrolysis of HNCO on oxide systems placed downstream. The yield of HNCO from NO is highest for Pt/SiO2 exceeding 35% at 315 degreesC with a standard 2800/3400/1200 ppm NO/CO/H-2 mixture. Hydrogen consumption is complete at 220 degreesC with ammonia and water as major products, but above 260 degreesC HNCO becomes the favoured product with some arising from NH3. Hydrolysis of HNCO to NH3 and CO2 takes over once all NO has been consumed. The extent of hydrolysis is increased somewhat if additional SiO2 is placed downstream. Other oxide systems-CeO2/SiO2, BaO/SiO2, CeO2/Al2O3, and CeO2-ZrO2-give complete hydrolysis to the extent of the available water at 315 degreesC, and no HNCO remains if additional water is included in the feed. Hydrogen consumption during the NO + CO + H-2 reaction commences at the lowest temperature on the Pd/SiO2, and for 100 degreesC from the onset temperature of 130 degreesC the reaction can be largely described as the sum of the NO + H-2 and NO + CO ones. Formation of HNCO commences at 235 degreesC, with a maximum yield of 20% at 300 degreesC. It appears to arise solely through utilisation of NH3 made as a side-product to the NO + H-2 reaction. Rh/SiO2 is much less active than Pt/SiO2 and Pd/SiO2 for the NO + H-2 reaction, but more active for the NO + CO and NO + CO + H-2 ones. The latter exhibits a small sharp peak in HNCO formation, but the maximum yield is only 14% and this coincides with total consumption of NO. Considerable ammonia is formed at higher temperatures, even though none is produced during the NO + H-2 reaction. HNCO is believed to arise on each metal through the combination of surface hydrogen atoms with metal-bound NCO groups which exist in small numbers when N atoms are located adjacent to adsorbed CO molecules. The differences in behaviour between the metals can be rationalised in terms of the relative strengths of adsorption of CO and NO, and the temperature difference between total consumption of H-2 and NO. If the difference is large, then HNCO can be produced from ammonia as well as hydrogen. A general conclusion is that, although some HNCO might be generated on platinum group metal particles within the pores of three-way vehicle catalysts during warm-up, the rapidity of hydrolysis on oxide washcoats with water in large excess is so great that no HNCO would ever emerge. Only the hydrolysis products, NH3 and CO2, will be seen.