The selective catalytic reduction (SCR) of NO by C 2 H 5 OH and IPA (isopropyl alcohol) over Ag/Al 2 O 3 was studied in the presence of SO 2 . The results showed that C 2 H 5 OH performs much more effectively in the presence of SO 2 compared with IPA. In situ DRIFTS study did not show the formation of sulfate species on Ag/Al 2 O 3 , which might explain the high resistance of Ag/Al 2 O 3 to SO 2 in the SCR of NO by C 2 H 5 OH. In addition, the deactivation mechanism for the case using IPA as a reductant was also proposed based on the IR spectra results.