Manganese oxide plays an important role in redox transformation of arsenic in the aquatic environment. High levels of carbonate and Fe(II) were found as major coexistent ions with arsenic, but their influences on oxidation of arsenite (As(III)) to arsenate (As(V)) by manganese oxide were not fully understood. This work researched oxidation of As(III) on hydrous manganese oxide (HMO) and investigated the effects of carbonate and Fe(II).