V2O5-MoO3/TiO2 and V2O5-WO3/TiO2 monolithic catalysts were evaluated for selective catalytic reduction (SCR) of NOx with NH3 in the presence of SO2 and H2O. After a durability test at 220 degrees C for 30 h, the NOx. conversion of V2O5-MoO3/TiO2 increased from 70% to 88% while it decreased slightly over V2O5-WO3/TiO2. The catalysts were characterized by H-2 temperature-programmed reduction (TPR), temperature-programmed desorption (TPD) of NH3, thermo-gravimetric (TG) analysis, temperature-programmed decomposition (TPD) of deposits, temperature-programmed surface reaction (TPSR) of ammonium bisulfates with NO, surface-enhanced Raman spectroscopy (SERS), ultraviolet-visible (UV-vis) and diffuse reflection infrared Fourier transform (DRIFT) spectroscopy. There were fewer sulfates deposits on VMoTi-S catalyst due to lower SO2 oxidation which was related to less V-O-V bonds on the Mo-modified catalyst. The as-formed high proportioned stable tridentate sulfates on VMoTi-S catalyst acted as strong Bronsted acid sites, and more active polymeric vanadates remained on this catalyst to activate the absorbed NH4+. The separation of acid sites and active sites which facilitates -NH2 formation was suggested to be responsible for the promoted SCR activity of V2O5-MoO3/TiO2 during NH3 center dot SCR reaction.