High concentration of ammonium (NH₄⁺) often occurs in groundwater. In this study, the behavior of NH₄⁺ catalytic oxidation by a manganese oxide (MnO_x) filter was investigated systematically and the formation and evolution of the MnO_x films were characterized extensively. It was found that the MnO_x based filter could be successfully started up within 6 days with the NH₄⁺ removal efficiency increasing from 1 to 96%. After start-up, the removal efficiency of NH₄⁺ could keep almost constant under different operating conditions, implying the MnO_x filter has a good stability towards NH₄⁺ removal. Both X-ray diffraction (XRD) and Raman results suggested that MnO_x was related to hexagonal birnessite which has low crystallinity and small crystallite sizes. The X-ray photoelectron spectroscopy (XPS) spectra of the Mn2p taken on the surface of MnO_x revealed Mn(II), Mn(III) and Mn(IV) species existed in MnO_x materials and their atomic concentrations were 20.2%, 57.8% and 22%, respectively. More importantly, the catalysis of films towards NH₄⁺ oxidation was expected to be closely related to its feature structure. Specifically, the ability of manganese to cycle between +2, +3 and +4 oxidation states during catalysis was considered to play important roles in the NH₄⁺ conversion on MnO_x surface. Based on the above discussion, three major steps involve in NH₄⁺ catalytic oxidation by MnO_x were proposed, which are NH₄⁺ adsorption, catalytic oxidation, and desorption of reaction products.