An in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) study of the selective catalytic reduction (SCR) of NO with NH3 in the presence of O-2 was carried out over vanadia-based catalysts. Vanadia-based catalysts were prepared by impregnation method. The results indicated that the strength of Bronsted acidity positively increased with V2O5 loading, whereas Lewis acidity remained almost constant at a typical SCR operating temperature. Water vapor does not affect the adsorption of Lewis ammonia, but the formation of Bronsted acidity has to be accompanied by water vapor adsorption. Bronsted acidity is decreased with increasing temperature and disappeared at a temperature of around 500degreesC for all testing catalysts, while Lewis acidity slightly decreased with increasing temperature. The disappearance of Bronsted acidity at a high temperature was related to the loss of adsorbed water vapor.