Hydrothermal conversion of guaiacol, which is used as a model compound to represent lignin, was conducted at temperatures of 300-450 degrees C for very short residence times of 0.5-40 s at a pressure of 25 MPa using a continuous-flow apparatus designed to rapidly heat the system to the desired temperature. The yields of char, gaseous products, phenolic compounds (phenol, guaiacol, catechol, o-cresol, and m-cresol), and benzene were determined. Guaiacol conversion under hydrothermal conditions occurred rapidly. The formation of compounds containing multiple benzene rings exclusively in the supercritical region indicated that extreme temperatures are needed to produce complex compounds. Under subcritical conditions, however, mainly phenolic and nonphenolic single-benzene-ring compounds were produced. The multiple-benzene-ring compounds were an important indication of cross linking between the single-ring compounds to form higher-molecular-weight fragments. This is consistent with our previous study of lignin conversion under hydrothermal conditions, which indicated that the formation of chars under supercritical conditions occurred because of the cross-linking reaction between the reactive degradation fragments, resulting in the production of higher-molecular-weight fragments. A reaction network model was proposed and the kinetic parameters for the pathways of guaiacol decomposition were determined by assuming a first-order reaction. It was observed that the rate constant of the overall guaiacol decomposition obeyed Arrhenius behavior, but some of the reactions in the proposed network deviated from this in the supercritical region. Thus, the individual rate constants of each reaction in the network were evaluated for the purpose of classification into ionic or radical reactions.