Hydrogen is a clean burning energy source and auxiliary fuel used in a novel power generation system. This paper reports on the kinetics of hydrogen oxidation in supercritical H2O/CO2 mixtures at temperatures between 505 and 600 degrees C and at a pressure of 23.6 MPa. The experiments were conducted in an isothermal, plug-flow reactor operating under continuous steady state. Hydrogen and carbon dioxide was produced by supercritical water gasification (SCWG) of 2 wt% methanol solution and oxygen was produced by pyrolysis of hydrogen peroxide (H2O2) solution. Inlet hydrogen concentration was around 0.1 mol/L, with fuel equivalence ratios varying from 0.46 to 0.91; the hydrogen to oxygen ratio thus ranged in fuel-lean conditions. For the range of conditions studied, a rate expression was established. It was found that the oxidation kinetics were independent of oxygen concentration and exhibited a first-order dependence of on hydrogen concentration. Empirical Arrhenius formula was used to determine the rate constants and the activation energy was estimated to be 105 +/- 5 kJ/mol. This value was comparable with activation energies obtained in other similar works.