Sulfation occurs when commercial vanadia SCR catalysts are exposed to SO2-laden coal combustion flue gases. Effects of sulfation on the surface chemistry of vanadia/titania catalysts and SCR activity have not been adequately addressed in previously published work. In this work, in situ MR spectroscopy and post situ XPS investigations were performed during vanadia/titania catalyst sulfation under simulated coal combustion flue gas conditions. In situ FTIR spectroscopy combined with XPS analyses on fresh and sulfated TiO2, 2% and 5% V2O5/TiO2 indicate that sulfate does not form on vanadia sites but rather on titania sites. MR spectroscopy data show that sulfation inhibits NO adsorption in the presence of oxygen, but greatly enhances NH3 adsorption by generating additional Bronsted acid sites while reducing the concentration of Lewis acid sites. Observed increases in the intrinsic NO reduction activity of sulfated vanadia/titania catalysts (relative to fresh catalysts) are consistent with spectroscopy data showing that the sulfation enhances NO reduction activity by increasing the number of active sites without changing the activation energy or site acid strength. (C) 2009 Elsevier B.V. All rights reserved.