The reaction between NH₃ and NO in excess CO over silica-supported platinum, palladium, and rhodium has been investigated for temperatures from 100 to 450 °C. As found previously for the corresponding reactions of H₂/NO/CO mixtures, isocyanic acid (HNCO) is produced with each catalyst. With Pd/SiO₂, the peak yield when NH₃ is used is 46% based on the total nitrogen converted and 55% based on the hydrogen taken from NH₃, with the remaining hydrogen converted to water. The reaction over Pt/SiO₂ requires a somewhat higher temperature and is more efficient with a maximum HNCO yield of 70% based on nitrogen and 95% on hydrogen. The near-absence of water as a product correlates with the high activity of platinum for the production of HNCO from H₂O/NO/CO mixtures, which in turn is driven by activity for the water–gas shift reaction. The peak yield of HNCO from NH₃/NO/CO mixtures is much lower with Rh/SiO₂ (10% based on nitrogen), and, unlike the yield of the platinum and the palladium catalysts, it is less than that observed during the H₂ + NO + CO reaction (30%). In experiments using ¹⁵NO and ¹⁴NH₃, the ¹⁵N content of N₂, HNCO, and the end nitrogen of N₂O are similar, consistent with dissociation of both nitric oxide and ammonia to form a single surface pool of nitrogen atoms. However, neither dissociation is rapidly reversible, since there is little exchange of ¹⁴N from ammonia into unreacted ¹⁵NO, and significant formation of ¹⁵NH₃ is confined to Pt/SiO₂ at temperatures where it could be formed by hydrolysis of product H¹⁵NCO. It is concluded that HNCO is formed by the rapid pick-up of surface hydrogen atoms by metal-bound NCO groups existing in equilibrium with N atoms and surrounding CO molecules. The trends in product distribution and in activity between the three metals can be rationalised in terms of competition between NO and CO for surface sites with CO favoured on Pt, NO favoured on Rh, and Pd exhibiting intermediate characteristics.