MnO_x–CeO₂ is a potential substitute for commercial Pt-based diesel oxidation catalysts. To better investigate its active sites for NO oxidation reactions, a series of MnO_x–CeO₂ samples with low Mn doping amounts (≤5%) were synthesized using the incipient-wetness impregnation method. The catalysts were characterized by various techniques (XRD, BET, XPS and H₂-TPR). According to XRD and XPS, a trace amount of manganese enriched on the surface of CeO₂ exists in higher oxidation states. With the increase of Mn doping, more Mn²⁺ is introduced into the CeO₂ lattice. The interaction between the manganese oxides and ceria improves the redox properties of MnO_x and the surface oxygen of CeO₂. Combining turn over frequency with the initial reducibility from H₂-TPR quantitation, it is found that H₂ consumption from Mn⁴⁺/Mn³⁺ to Mn₃O₄ is equivalent to the active site for NO oxidation reactions. In conclusion, Mn⁴⁺/Mn³⁺ is the active species for NO oxidation reactions.