Oxidation states of Sb and As in marine ferromanganese oxides determined by X-ray absorption near-edge structure showed that both As and Sb are pentavalent in the ferromanganese oxides. The concentration ratios of As/Sb in the ferromanganese oxides are similar to those in seawater. There results are consistent with the fact that As and Sb behave similarly, especially under oxic conditions as suggested from other studies in soil-water and river-sediment systems.