The hydrothermal gasification of xylose, as a model substance for the main polysaccharide in hemi-celluloses, was carried out at high temperatures (up to 600 degrees C) and high pressures (up to 42.5 MPa) in the presence of an alkali catalyst with a constant reaction time of 3600 s. The comparison of the experimental results, such as the volume of the gas phase, composition of the gas, and the liquid products together with the total organic carbon content (TOC) provides the knowledge of the chemical reaction pathways of the xylose and their dependence on the reaction conditions, temperature, pressure, and catalyst. Both the gas and liquid phases were analyzed using chromatographic techniques. The major gaseous compounds produced were hydrogen, methane, carbon dioxide, carbon monoxide and C-2-C-4 hydrocarbons. The carbon gasification efficiencies were improved by increasing the temperature and decreasing the pressure and reached their maximum value (86%) at 20 MPa and 600 degrees C with the addition of K2CO3. The highest measured concentration of the retro aldol reaction product was acetic acid and the dehydration product was 5-methyl furfural. (C) 2016 Elsevier Ltd. All rights reserved.