The aim of this study was to determine the complete reaction network of xylose decomposition in sub- and supercritical water, including small-molecule intermediates, such as organic acids, which are thought to be the final intermediates in the formation of gaseous products. Solutions of xylose in water were heated under sub- and supercritical conditions in the temperature range of 350-450 degrees C in a continuous reactor at a controlled pressure of 25 MPa. The intermediates found in the liquid phase were xylulose, furfural, retro-aldol products (glyceraldehyde, glycolaldehyde, dihydroxyacetone, and formaldehyde), and organic acids (acetic and formic acids). The reaction types involved were classified according to Arrhenius behavior: the ionic reaction (not showing Arrhenius behavior in the supercritical region) and the free-radical reaction (showing Arrhenius behavior in the supercritical region). Formic acid was the final intermediate before gasification, while acetic acid and formaldehyde were not gasified in the temperature range of this study.