Manganese oxide as a benign and low-cost oxidant has been widely investigated for the oxidation of As (III) to As(V), but its As(III) oxidation rate is easily hindered due to the thermodynamically favorable adsorption of the As(V) species produced and coexisting ions. Herein, we develop a novel strategy of tremendously increasing the As(III) oxidation activity via tuning the concentration of oxygen vacancy defects (OVD) in cryptomelane-type manganese oxides (OMS-2). The result of the batch experiment of the As(III) oxidation on OMS-2 shows that increasing the concentration of oxygen vacancy defects in OMS-2 remarkably increases the specific As(III) oxidation rate from 0.12 to 0.22 mu mol m(-2) min(-1), and dramatically reduces the unfavorable effect of the As(V) adsorption on the As(III) oxidation activity. The origin of the tremendous effect of OVD in OMS-2 on the As(III) oxidation activity is experimentally and theoretically investigated by the adsorption of the As(III) and As(V) species on OMS-2, the point of zero charge (pH(PZC)), the evolution of the concentration of Mn2+ ions dissolved in the solution, and density functional theory (DFT) calculations. The results reveal that the presence of OVD in OMS-2 not only makes the adsorption of As(III) on OMS-2 more thermodynamically favorable, but also accelerates the charge transfer from As(III) to Mn atom, thus significantly improving the As(III) oxidation activity. (C) 2016 Elsevier B.V. All rights reserved.