The performances of two model HC-SCR catalysts, 5% Cu/ZSM-5 (Si/Al ratio of 30) and 2% Ag/Al2O3, are compared and contrasted under a variety of different operating conditions. We have examined the activity of these catalysts by using a wide range of hydrocarbons. The activity of the Cu/ZSM-5 catalyst is not significantly affected by the nature of the hydrocarbon species, whereas the Ag catalyst shows a strong dependence. The Ag catalyst displays poorer low-temperature activity with propene than with longer chain hydrocarbons, such as decane. The use of diesel fuel with the Ag catalyst induces deactivation at lower temperatures, because of coke deposition. The Ag catalyst is strongly deactivated by sulphur. The Cu/ZSM-5 catalyst is much more resistant to chemical deactivation. The Ag/Al2O3 catalyst is capable of withstanding high-temperature ageing conditions, whereas the Cu/ZSM-5 catalyst is significantly deactivated after such ageing because of a combination of Cu sintering and zeolite dealumination. (C) 2004 Elsevier Inc. All rights reserved.