We oxidized methanol in supercritical water at 500 degrees C to explore the influence of the water concentration (or density) on the kinetics. The rate increased as the water concentration increased from 1.8 to 5.7 mol/L. This effect of water density on the kinetics observed experimentally was quantitatively reproduced by a previously validated mechanism-based, detailed chemical kinetics model. In this model, reactions of OH radicals with methanol were the fastest methanol removal steps. The rates of these removal steps increased with water density at 500 degrees C because the OH radical concentration increased. The OH radical concentration increased with density because the rates of the steps H + H2O = OH + H-2 and CH3 + H2O = OH + CH4, which produce OH radicals, increased. Thus, the main role of water in accelerating methanol oxidation kinetics at 500 degrees C is as a hydrogen donor to a radical (R) in steps such as R + H2O = OH + RH. This system provides a striking example of SCW being involved on the molecular level in the free-radical oxidation as a reactant in elementary steps.