A detailed chemical kinetic model for high-pressure methanol oxidation in the gas phase and supercritical water (SCW) has been developed, updating kinetic parameters for key reactions. Based on a careful analysis of ignition delay measurements for methanol at high pressures in shock tubes, a rate constant has been derived for reaction CH3OH + HO2. The rate constant is significantly higher than recent values calculated by high-level theory. Comprehensive validation of the model was conducted, comparing predictions against experimental data over a wide range of conditions in both the gas phase and SCW. Species measurements for methanol oxidation in high-pressure gas-phase flow reactors (20-100 bar) and ignition delay times from rapid compression machines and shock tubes (12-50 bar) were reproduced well by the model. Also, modeling predictions of SCWO of methanol were generally in agreement with the experiment, with discrepancies attributed mostly to experimental artifacts such as hot spots and nonideal hydrodynamics. Based on the model, a comparative kinetic analysis was conducted to explore the characteristic similarities and differences of methanol oxidation under the two conditions.