The hydrothermal system at Vulcano, Aeolian Islands (Italy), is home to a wide variety of thermophilic, chemolithoautotrophic archaea and bacteria. As observed in laboratory growth studies, these organisms may use an array of terminal electron acceptors (TEAs), including O2, , Fe(III), , elemental sulphur and CO2; electron donors include H2, , Fe2+, H2S and CH4. Concentrations of inorganic aqueous species and gases were measured in 10 hydrothermal fluids from seeps, wells and vents on Vulcano. These data were combined with standard Gibbs free energies () to calculate overall Gibbs free energies (ΔGr) of 90 redox reactions that involve 16 inorganic N-, S-, C-, Fe-, H- and O-bearing compounds. It is shown that oxidation reactions with O2 as the TEA release significantly more energy (normalized per electron transferred) than most anaerobic oxidation reactions, but the energy yield is comparable or even higher for several reactions in which , or Fe(III) serves as the TEA. For example, the oxidation of CH4 to CO2 coupled to the reduction of Fe(III) in magnetite to Fe2+ releases between 94 and 123 kJ/mol e−, depending on the site. By comparison, the aerobic oxidation of H2 or reduced inorganic N-, S-, C- and Fe-bearing compounds generally yields between 70 and 100 kJ/mol e−. It is further shown that the energy yield from the reduction of elemental sulphur to H2S is relatively low (8–19 kJ/mol e−) despite being a very common metabolism among thermophiles. In addition, for many of the 90 reactions evaluated at each of the 10 sites, values of ΔGr tend to cluster with differences < 20 kJ/mol e−. However, large differences in ΔGr (up to ∼ 60 kJ/mol e−) are observed in Fe redox reactions, due largely to considerable variations in Fe2+, H+ and H2 concentrations. In fact, at the sites investigated, most variations in ΔGr arise from differences in composition and not in temperature.