H2O inhibits the SCR performance of the Ce-Cu-Ti (CCT) oxide catalyst at low temperatures, while it promotes SCR performance at high temperatures above 300 degrees C. A combination of experiments and DFT calculations was applied to study this phenomenon. NH3 adsorption profiles showed that the presence of H2O would greatly decrease the NH3 adsorption amount, especially the weakly adsorbed NH3 part. The DFT calculations of H2O and NH3 adsorption on the CCT catalyst showed that the presence of H2O would compete with NH3 to be adsorbed on the catalyst. NH3 oxidation experiments indicated that NH3 oxidation at high temperatures decreases with the addition of 10% H2O. The DFT calculation also indicated that H2O would inhibit NH3 oxidation. The inhibition effect of H2O at low temperatures could be attributed to the competing adsorption of H2O with NH3. The promotional effect of H2O at high temperatures was due to the inhibition of NH3 oxidation. (c) 2013 Elsevier B.V. All rights reserved.