Selective catalytic reduction (SCR) of NO with NH3 was investigated using TiO2-supported Mn oxide catalysts prepared by wet impregnation at different MnO (x) loadings and reduction temperatures. The structures, valence states, and physicochemical properties were characterized using Brunauer-Emmett-Teller, X-ray diffraction, Raman, and X-ray photoelectron methods. The 20 wt% MnO (x) /TiO2 catalyst, which was rich in Mn4+ species, showed the best SCR activity and N-2 selectivity at low temperatures. As the reduction temperature increased, the Mn valence states gradually changed from Mn4+ to Mn2+. When the valence state was lowered (Mn4+ to Mn2+), the SCR activity decreased significantly at low temperatures. H-2 shut-off tests confirmed that the activity of the reduced catalyst was not fully recovered. For superior SCR activity at low temperatures, the MnO (x) species on the MnO (x) /TiO2 catalyst surface should be kept as Mn4+.