Reaction kinetics of methanol oxidation in supercritical water at high pressure condition (420 C; 34-100 MPa; rho = 300-660 kg/m(3)) was investigated. Pseudo-first order rate constant for methanol decomposition increased with increasing water density. Effects of supercritical water on the reaction kinetics were investigated using a detailed chemical kinetics model. Incorporating the effect of diffusion in a reduced model revealed that overall kinetics for SCWO of methanol is not diffusion-limited. Roles of water as a reactant were also investigated. The dependence of sensitivity coefficient for methanol concentration and rate of production of OH radical on water density indicated that a reaction, HO(2) + H(2)O = OH + H(2)O(2), enhanced the OH radical production and thereby facilitated the decomposition of methanol. It is presumed that concentration of key radicals could be controlled by varying pressure intensively. (C) 2011 Elsevier B.V. All rights reserved.