On-line infrared spectroscopy has been used to show that isocyanic acid, HNCO, is a substantial primary product of the reaction between NO, CO and H-2 over aPt/SiO2 catalyst. At 230 degrees C it accounts for more than 40% of the CO converted. No isocyanic acid is seen when Al2O3 is placed downstream of Pt/SiO2, or when using a Pt/Al2O3 catalyst under the same conditions, due to rapid hydrolysis of HNCO on the alumina. Isocyanic acid may exist as a trace intermediate in automobile catalytic converters during their warm-up phase but it is unlikely to emerge from the pore system of alumina-based catalysts.