A new Ag/Al2O3 catalyst for removing NOx in diesel engine exhaust gas was developed. The influence of SO2 on the reduction of lean NOx by ethanol over the Ag/Al2O3 catalyst was evaluated in simulated diesel exhaust and characterized using TPD, XRD, XPS, SEM and BET measurements. The Ag/Al2O3 catalyst was highly active for the reduction of NOx with ethanol in the presence of SO2 although the reduction of NOx is suppressed at lower temperatures. The activity for NOx reduction is high even on the Ag/Al2O3 catalyst exposed to a SO2 (200 ppm)/O-2 (10%)/H2O (10%) flow for 20 h at 723 K and comparable to that on the fresh Ag/Al2O3 catalyst. No crystallized Ag metal and Ag compounds were formed by the SO2/O-2/H2O exposure. On the other hand, crystallized Ag2SO4 was easily formed when the Ag/Al2O3 catalyst was exposed to a SO2 (200 ppm)/O-2 (10%)/NO (800 ppm)/H2O (10%) Bow for 10 h at 723 K. XRD, SEM and XPS studies showed that the formation of crystallized Ag2SO4 results in growing of Ag particles in larger size and lowering the surface content of Ag particles. In addition, the specific surface area of the Ag/Al2O3 catalyst decreases from 221 to 193 m(2)/g. Although the dispersion of Ag particles was decreased by the formation of Ag2SO4, the activity for the reduction of lean NOx was, remarkably, not affected. This suggests that the Ag-alumina sites created by the Ag2SO4 formation are still active for the lean catalytic reduction of NOx.