Steady-state activity of Pt-ZSM-5 catalysts has been investigated experimentally for the NO + C2H4 + O-2 reaction under highly oxidizing conditions, typical of lean-bum gasoline engine exhaust. Effects of temperature, space velocity, feed concentration, Pt loading and water vapor on the catalytic activity have been examined using a packed-bed laboratory reactor. The catalytic activity of Pt-ZSM-5 is discussed in comparison with that of Cu-ZSM-5 and Pt/Al2O3. Results show that Pt-ZSM-5 catalysts are much more active than Cu-ZSM-5 catalysts for lean-NOx reduction at low temperatures, while the kinetic behavior of Pt/Al2O3 is very similar to that of Pt-ZSM-5. Conversion of both NO and C2H4 during the NO + C2H4 + O-2 reaction over Pt-ZSM-5 around the reaction lightoff temperature is strongly inhibited by the presence of NO. The NO/C2H4 ratio in the feedstream is an important factor determining the NO reduction activity of the catalyst, and there exists an optimum value of this ratio for a maximum conversion of NO. Based on the steady-state NO conversion data, a correlation between the reactor performance and the feed concentration has been developed, and the feasibility of Pt-based catalysts for lean-NOx reduction is discussed in terms of their activity, selectivity and durability.