The reaction of NO with excess CO in the presence of water vapour has been studied for silica- and alumina-supported platinum. With Pt/SiO2, HNCO is the dominant nitrogen-containing product between 300 and 400degreesC, with a yield of up to 50%. Higher temperatures favour NH3 formation due to HNCO hydrolysis and shifts in surface populations that can be interpreted in terms of the NO + CO and water-gas shift reactions. When excess water is present, the hydrolysis activity of Pt/Al2O3 is sufficient to convert all HNCO to NH3. The yield of NH3 exceeds 75% at temperatures above 280 degreesC, where conversion of NO becomes complete. Insufficient water leads to the formation of Al-NCO groups and a loss of activity that can be reversed by additional water. Pt/CeO2 behaves similarly to Pt/Al2O3. Pd/SiO2 also generates some HNCO, but the concentrations made are much less than that over Pt/SiO2 due to a higher activity for the NO + CO reaction and a lower activity for the CO + H2O one. (C) 2003 Elsevier B.V. All rights reserved.