Contamination of groundwater by arsenic in the form of arsenite As(III) and arsenate As(V) has become an environmental problem globally. In this study, the competitive adsorption of As(III) and As(V) on gamma-Al2O3 was examined, and As(V) was found to be preferentially adsorbed from an aqueous solution of As(III) and As(V). To improve the removal of As(III) from aqueous solution, MnO2 was mixed with the adsorbent as an oxidizing agent to convert inorganic As(III) to As(V), and the effects of pH and the amount of added MnO2 on the removal of As(III) were examined. The addition of MnO2 increased the removal of As(III) from 20 to 90% at pH 3. The dissolution of MnO2 through oxidation of As(III) decreased with increasing pH. Above pH 6, As(III) was oxidized and adsorbed without Mn2+ remaining in solution, probably due to the adsorption of Mn2+ on the surface of MnO2. The oxidation rate was slow compared with the adsorption rate of As(III) and As(V) on gamma-Al2O3.