A study of the inter-relationship between moisture, vanadia content, operation temperature, and surface acidity for a selective catalytic reduction (SCR) process for nitric oxide (NO) removal was conducted. The effects of operating parameters on the roles of Brønsted and Lewis acids acting in the SCR reaction were specified by the observed NO removal and the spectra of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The results indicated that H2O competed with NH3 molecules to be adsorbed on the pure titania catalyst that contained only Lewis acid sites. However, the competition did not occur over a vanadia-based catalyst on which both Brønsted and Lewis acid sites appeared in the presence of H 2O. As compared to that under dry conditions where only Lewis acid sites were found, the catalytic activity was lower under moisturized conditions even though both acids existed and there are fewer reaction sites available. It is concluded that surface acid sites are varied according to different operation conditions. Lewis acidity was dominant under the condition of low vanadia content and in the absence of H2O molecules. It is also less sensitive to temperature. In contrast, Brønsted acidity was dominant under the condition of high vanadia content and in the presence of H 2O molecules, and it is highly affected by temperature.