The kinetics of hydrogen oxidation in supercritical water were determined in an hermal, ug-flow reactor at temperatures between 495 and 600°C and at a pressure of 246 bars. Inlet hydrogen and oxygen concentrations ranged from 0.4 × 10−6to 4.6 × 10−6mol/cm3, with fuel equivalence ratios varying from 0.04 to 4.0. Over the range of conditions studied, the kinetics were independent of oxygen concentration and exhibited a first-order dependence on hydrogen concentration, with an activation energy of 372±34 kJ/mol and a pronounced induction time. An elementary reaction model, based on existing gas-phase models and modified to account for the high-pressure environment, was able to reproduce closely the experimental results. including the overall concentration dependencies (reaction orders) and activation energy. Based on the model. important elementary reactions were identified and details of the oxidation mechanism were inferred.