The active sites of powder and monolith V2O5-WO3/TiO2 catalysts prepared on commercial titania supports were studied for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) by ammonia (NH3) while varying vanadium oxide (V2O5) coverage. Four different types of commercial titania (DT-51, DT-56, G-5, and S-10) were used in this study. It was observed that the SCR activity of powder V2O5-WO3/TiO2 is highly dependent on the vanadium coverage. The catalytic activity increased with increasing V2O5 coverage and reached a maximum between one-half and one monolayer of vanadia. However, NOx conversion decreased at high V2O5 coverage. The catalysts were characterized by temperature-programmed desorption of NH3 (NH3 TPD), temperature-programmed reduction by hydrogen (H-2 TPR), oxygen chemisorption, and N-2 adsorption to evaluate the active sites responsible for high SCR activity. The spectroscopic studies revealed that the rapid decrease in SCR activity at higher vanadia coverage is due to the decrease in the reducibility and dispersion of vanadium species. This synthetic learning was extended to investigate the active sites in extruded monolith V2O5-WO3/TiO2 catalysts. A good correlation between redox sites and activity for powder and monolith V2O5-WO3/TiO2 catalysts was found. [GRAPHICS] .