The Mn/TiO2 and a series of Mn-Ni/TiO2 catalysts were prepared by adopting incipient wetness technique and investigated for the low-temperature SCR of NO with NH3 in the presence of excess oxygen. Our XPS results illustrated that the MnO2 is the dominant phase with respect to the Mn2O3 phase (Mn4+/Mn3+ = 22.31, 96%), thus leading to a large number of Mn4+ species (Mn4+/Ti) over the titania support for the Mn-Ni(0.4)/TiO2 catalyst. It is remarkable to note that the SCR performance of all the nickel-doped Mn/TiO2 catalysts is accurately associated with the surface Mn4+ concentrations. The co-doping of nickel into the Mn/TiO2 with 0.4 Ni/Mn atomic ratio promotes the formation of surface MnO2 phase and inhibits the formation of surface Mn2O3 sites. Our TPR results revealed that the addition of nickel oxide to titania-supported manganese results in the stabilization of the former in the form of MnO2 rather than Mn2O3. Our TPR data results are in agreement with XPS results that the absence of the high-temperature (736 K) peak indicates that the dominant phase in the Mn-Ni/TiO2 catalysts is MnO2. The low-temperature reduction peak is shifted to much lower temperatures in nickel-doped Mn/TiO2 catalysts. This increase in reducibility and the extremely dominant MnO2 phase seem to be the reason for the high SCR activity of the Mn-Ni/TiO2 catalysts. Published by Elsevier Inc.