The purpose of this study was to elucidate the decomposition characteristics of xylose, a model compound for hemicellulose, in subcritical and supercritical water. The experiment was carried out at temperatures of 300-450 degrees C, a pressure of 25 MPa, and a residence time of less than 7 s; xylose decomposed rapidly, but it was still detected at a temperature of 300 degrees C. Furfural and retro-aldol condensation products were found to be the major liquid intermediates. A reaction network was proposed and the kinetics parameters of all reactions were calculated on the basis of data fitting, assuming that all reactions are first-order. Finally, the temperature effect was used to classify the reactions as radical reactions (showing Arrhenius behavior in the supercritical region) or as ionic reactions (not showing Arrhenius behavior in the supercritical region).