Predictions of bioavailable energy can be made through calculation of geochemical disequilibria. The geochemical model of McCollom and Shock  of mid-ocean ridge black smoker systems was tested against microbiological measurements collected in these environments. This model was modified in this paper to include hydrogen oxidation, as there was evidence that hydrogen—oxidizing microorganisms inhabit black smoker environments. The sharp distinction between high tempearture, anoxic environments and low temperature, oxic environments in the model of McCollom and Shock  disappeared in the new model; both reductive and oxidative metabolic strategies provided energy in low-and hightemperature environments. Future directions for model modification are explored, in light of potential subseafloor biotopes.