Hydrogen produced by serpentinization has the potential to fuel subsurface microbial metabolisms. In the serpentinizing subsurface, the solids comprise ultramafic parent rocks derived from the Earth's mantle, serpentine minerals, veins of hydroxides, and accessory magnetite and/or other metal-rich grains. Fluid that occurs with these solids is altered seawater and/or meteoric water and is predicted to be reducing. Hydrogen, a powerful reducing agent, is generated when Fe2 in Fe(OH)2 is oxidized to magnetite, coupled to the reduction of water. Theoretical considerations and experimental work suggest that serpentinization may generate fluid H2 concentrations as high as ≈75 millimolar, and that related seeps on land should have ≈300 micromolar. Field observations have shown that submarine serpentinizing seeps contain fluid H2 concentrations of 1 to 15 millimolar H2, subseafloor sediments have ≈7–100 nanomolar H2, and thermal springs have ≈13 nanomolar H2. Fluid H2 has the potential to drive a variety of metabolic processes in oxygen- and organic carbon-deprived environments, such that considerable interest has developed in the potential of serpentinizing systems as an abode of deep subsurface life. Based on empirical parameters, we have modeled the free-energy change for an array of metabolic reactions that may be associated with serpentinization, and find that metabolic niches do exist for methanogenesis, ferric iron reduction, sulfate reduction, and nitrate reduction, given environmentally realistic fluid chemistries.