Oxidation in a supercritical water environment is an efficient method for treating wastes without formation of harmful byproducts. Complete oxidation of organics can be limited by conversion of carbon monoxide to carbon dioxide. The kinetics of carbon monoxide oxidation has been studied previously (Helling, R. K.; Tester, J. W. Energy Fuels 1987, 1, 417), but efforts to establish kinetic parameters for the direct oxidation pathway (CO + 1/2O2 → CO2) were complicated by the reaction of carbon monoxide with water via the water-gas shift reaction pathway (CO + H2O CO2 + H2) during preheating of the reactor feeds. The kinetics of carbon monoxide oxidation has been reexamined in an updated experimental apparatus with improved temperature measurement and hydrogen detection capabilities, and the studied range of O2/CO feed ratios has been extended to the substoichiometric regime. Using the results of heat-transfer experiments, the temperature profiles within the feed preheater were established. The experimentally determined temperature profiles compared favorably with profiles predicted by conventional heat-transfer correlations. These temperature profiles were used in determining new kinetic parameters for the water-gas shift pathway and for the direct oxidation of carbon monoxide in supercritical water. The two kinetic pathways can apparently be treated as separable under the conditions of this study. Regressed kinetic rate forms were similar to those obtained in the earlier study. The new oxidation data exhibit a fractionalorder dependence on oxygen concentration, consistent with gas-phase studies of carbon monoxide oxidation.