H₂ thresholds for microbial respiration of arsenate (As(V)) were investigated in a pure culture of Sulfurospirillum arsenophilum. H₂ was consumed to threshold concentrations of 0.03-0.09 nmol/L with As(V) as terminal electron acceptor, allowing for a Gibbs free-energy yield of 36-41 kJ per mol of reaction. These thresholds are among the lowest measured for anaerobic respirers and fall into the range of denitrifiers or Fe(III)-reducers. In sediments from an arsenic-contaminated aquifer in the Red River flood plain, Vietnam, H₂ levels decreased to 0.4-2 nmol/L when As(V) was added under anoxic conditions. When As(V) was depleted, H₂ concentrations rebounded by a factor of 10, a level similar to that observed in arsenic-free controls. The sediment-associated microbial population completely reduced millimolar levels of As(V) to arsenite (As(III)) within a few days. The rate of As(V)-reduction was essentially the same in sediments amended with a pure culture of S. arsenophilum. These findings together with a review of observed H₂ threshold and steady-state values suggest that microbial As(V)-respirers have a competitive advantage over several other anaerobic respirers through their ability to thrive at low H₂ levels.