We investigate the deactivation effects of potassium on CeMoTiOx for the selective catalytic reaction of NOx with NH3. CeMoTiOx catalyst exhibited high SCR activity and N-2 selectivity during the SCR reaction. Potassium is found to inhibit the SCR activity of CeMoTiOx, but the poisoned CeMoTiOx can be regenerated. Characterizations, including H-2-temperature programmed reduction, temperature programmed desorption, and in situ diffuse reflectance infrared Fourier transform spectroscopy, revealed that potassium suppresses the reduction and adsorption capacity of CeMoTiOx. According to the separated oxidation results, potassium suppresses the oxidation of NO to NO2 and promotes the oxidation of NH3 to NOx and N2O; this may be the main reason for the decrease of low-temperature SCR activity and N-2 selectivity respectively over CeMoTiOx. In addition, kinetic study demonstrated that potassium has an inhibitory effect on SCR (4NH(3) + 4NO + O-2 -> 4N(2) + 6H(2)O) reaction rates, while it enhances both NSCR (4NH(3) + 4NO + 3O(2) -> 4N(2)O + 6H(2)O) and CO (4NH(3) + 5O(2) -> 4NO + 6H(2)O) reaction rates in the high temperature region.