Results and analysis of the oxidation of methane in supercritical water by oxygen over a pressure range from 35 to 270 bar and a temperature range from 390 to 440 degrees C are presented. Raman spectroscopy is used as an in situ diagnostic to monitor the concentration of methane, oxygen, carbon monoxide, and carbon dioxide in a constant volume reactor. Reaction orders with respect to methane and oxygen at 270 bar and at methane concentrations near 0.1 mol/L are close to two and zero, respectively. A nonmonotonic dependence of reaction rates on water concentration is observed. With temperature and initial concentrations held constant, methane consumption rates first increase with water concentration but reach a maximum near 5 mol/L. Further increases in water concentration lead to a sharp decrease in the rate of methane consumption. An existing, high-pressure elementary reaction mechanism reproduces this downturn in rates and provides insight into the reasons for this behavior.