The Ag-Pd/Al2O3 catalyst showed a higher NO, conversion than Ag/Al2O3 in the selective catalytic reduction (SCR) of NQ, by C3H6 especially at temperatures ranging from 573 to 773 K. However, the presence Of SO2 in the feed gas decreased the activity of NQ, reduction over AgPd/Al2O3 more greatly than that over Ag/Al2O3. The sulphate species formed on the poisoned Ag-Pd/Al2O3 and Ag/Al2O3 were characterized by BET, inductively couple plasma optical emission spectrometer (ICP-OES), XRD, temperature programmed desorption (TPD) and in situ diffuse reflectance infra-red fourier transform spectroscopy (DRIFFS) methods. Two kinds of sulphate species were found on the SO2 poisoned AgPd/Al2O3 catalyst, and the formation of nitrate was blocked by the surface sulphate species. The addition of Pd into Ag/Al2O3 promoted the formation of sulphate on the Ag sites in the presence of SO2, which was considered to be the possible reason for the weak sulphur resistance of Ag-Pd/Al2O3. (c) 2006 Elsevier B.V. All rights reserved.