The intermediates and the mechanism of the SCR reaction of NO with NH3 on TiO2 sulfate have been studied by Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), FT-IR and NH3-TPD. In the presence of ammonia on the surface, NO adsorbed as NO- species are enhanced and then oxidized to NO2 and NO3- by oxygen either from structure or NO. The formation of NO3- species perturbs the adsorption of protonated ammonia and induces its removal as N-2 at 300 degrees C. The result obtained from FT-LR shows that the NH3 species adsorbed on Lewis acid site remain as main species at high temperature. Accordingly, a Langmuir-Hinshelwood mechanism on Lewis acid site is suggested. (C) 2000 Elsevier Science B.V. All rights reserved.