The intermediates on Ag/Al2O3 during the C2H5OH-SCR of NO were studied using in situ DRIFTS, TPD, and DFT calculations. A ring structure enolic species was found on the surface of Ag/Al2O3 during the C2H5OH-SCR of NO by in situ DRIFTS, and this finding was supported by DFT calculations. The adsorbed enolic species is more prone to react with NO + O-2 on Ag/Al2O3 than acetate is, and plays a crucial role in the formation of NCO which is a key intermediate during the SCR of NO. TPD results suggest that the enolic species is derived from the aldol condensation reaction of a partial oxidation product Of C2H5OH (such as CH3CHO) on the surface of Ag/Al2O3. The results of DFT calculations are in good agreement with the DRIFTS spectra, and support our conjecture that the enolic species has a ring structure. (C) 2004 Elsevier B.V. All rights reserved.