Hydrothermal experiments were conducted to evaluate the kinetics of H₂₍ₐq₎ oxidation in the homogeneous H₂–O₂–H₂O system at conditions reflecting subsurface/near-seafloor hydrothermal environments (55–250°C and 242–497bar). The kinetics of the water-forming reaction that controls the fundamental equilibrium between dissolved H₂₍ₐq₎ and O₂₍ₐq₎, are expected to impose significant constraints on the redox gradients that develop when mixing occurs between oxygenated seawater and high-temperature anoxic vent fluid at near-seafloor conditions. Experimental data indicate that, indeed, the kinetics of H₂₍ₐq₎–O₂₍ₐq₎ equilibrium become slower with decreasing temperature, allowing excess H₂₍ₐq₎ to remain in solution. Sluggish reaction rates of H₂₍ₐq₎ oxidation suggest that active microbial populations in near-seafloor and subsurface environments could potentially utilize both H₂₍ₐq₎ and O₂₍ₐq₎, even at temperatures lower than 40°C due to H₂₍ₐq₎ persistence in the seawater/vent fluid mixtures. For these H₂–O₂ disequilibrium conditions, redox gradients along the seawater/hydrothermal fluid mixing interface are not sharp and microbially-mediated H₂₍ₐq₎ oxidation coupled with a lack of other electron acceptors (e.g. nitrate) could provide an important energy source available at low-temperature diffuse flow vent sites. More importantly, when H₂₍ₐq₎–O₂₍ₐq₎ disequilibrium conditions apply, formation of metastable hydrogen peroxide is observed. The yield of H₂O₂₍ₐq₎ synthesis appears to be enhanced under conditions of elevated H₂₍ₐq₎/O₂₍ₐq₎ molar ratios that correspond to abundant H₂₍ₐq₎ concentrations. Formation of metastable H₂O₂ is expected to affect the distribution of dissolved organic carbon (DOC) owing to the existence of an additional strong oxidizing agent. Oxidation of magnetite and/or Fe⁺⁺ by hydrogen peroxide could also induce formation of metastable hydroxyl radicals (•OH) through Fenton-type reactions, further broadening the implications of hydrogen peroxide in hydrothermal environments.