Pt-USY was used for the selective catalytic reduction of NO, with hydrocarbons in the: presence of excess oxygen. The catalyst was prepared by an ion-exchange method and characterized by XRD, TEM, CO chemisorption, and Ar adsorption at 87 K. The platinum particle size distribution was found to be broad (2-20 nm), with no apparent sintering of the active phase during the HC-SCR process after 25 h time-on-stream. Generally, large metal clusters (>15 nm) are situated at the external surface of the zeolite, while the smaller ones are located in the pores of the support. Pt-USY shows an excellent activity in the deNO(x) reaction (molar NOx conversion 90% at 475 K) with propene as the reductant in 5 kPa O-2, as well as stable operation during time-on-stream. Propane only yields a low NOx conversion compared to propene. The presence of high oxygen contents (5-10 kPa O-2) slightly inhibits the reaction. No significant decrease in deNO(x) activity was observed at high space velocities (up to 100,000 h(-1)). The presence of SO2 and H2O in the feed stream did not significantly affect the deNO(x) activity. Pt-USY performs better under lean-burn conditions than other Pt-catalysts supported on e.g. ZSM-5, Al2O3, or SiO2 The selectivity to N-2 was similar to the other Pt-based catalysts (similar to 30%), the other major product being N2O. (C) 2001 Elsevier Science B.V. All rights reserved.