It has been demonstrated that the sulfate-reducing bacterium Desulfotomaculum orientis can be grown in batch cultures on elemental hydrogen (H-2) as an energy source, carbon dioxide (CO2) as a carbon source, and sulfur dioxide (SO2) as a terminal electron acceptor. At sufficiently high H-2 partial pressures, complete reduction of SO2 to H2S was observed with 1-2 s of gas-liquid contact time. The maximum specific activity for SO2 reduction was 6.5 mmol SO2/h-g of total biomass protein. The stoichiometry of SO2 reduction with H-2 as the electron donor has been determined under these conditions. At low H-2 partial pressures, SO2 was both oxidized to sulfate and reduced to hydrogen sulfide (H2S).