Mobilization of arsenic (As) in the subsurface environment can result in elevated concentrations of As in groundwater and potential human exposure and adverse health effects. Natural attenuation (i.e., sequestration) of As may, under appropriate geochemical conditions, serve to limit human exposure to As. The effectiveness of As sequestration by sorption, co-precipitation, and/or precipitation can be strongly influenced by redox conditions, which can control the solubility of sorbent phases and the stability of As-containing solids. The redox transformation of As between the +III and +V oxidation states can also affect the extent of As sorption. The effect of amendment with synthetic manganese (Mn) oxide birnessite (nominally MnO2) on As sequestration in a sediment suspension was examined in the absence and presence of iron (Fe) added as Fe(II). In the absence of Fe(II), the extent of As(III) oxidation to As(V) increased with increasing birnessite amendment, but As sequestration was not increased. In the presence of Fe(II), however, As sequestration did increase with increasing birnessite amendment. Concurrently, Fe(II) was also sequestered, and the Fe(III) content of the solid phase was observed to increase, suggesting that the oxidative precipitation of an Fe(III) oxyhydroxide phase plays an important role in As sequestration. These results suggest that amendment with Mn(III, IV) oxides could be an effective way to augment natural attenuation of As in cases where As-contaminated groundwater also contains elevated concentrations of Fe(II). © 2009 Springer Science+Business Media B.V.