Two kinetic models describing supercritical water gasification of xylose at reaction temperatures from 450 degrees C to 650 degrees C and 250bar were developed. Reaction rate constants were non-linearly estimated from product yield vs. residence time data by sum of the least squares method. The xylose decomposition kinetic model uses a detailed reaction mechanism to predict liquid intermediate production and gasification rates, whereas the xylose gasification kinetic model uses a simplified reaction mechanism to better predict gas yield and gas composition at conditions where gasification is dominant. Both models assume the gas phase reactions are in thermodynamic equilibrium, however, the gasification kinetic model accounts for non-ideal interactions in the reacting fluid by incorporating the fugacity of the gas phase species into the model using the Peng-Robinson equation of state. Major gas products were CO2. H-2, CH4, CO, and C2H6. The highest measured concentration of liquid intermediate products were acetic and propanoic acid. Finally, an analysis of gas composition and gas yields for concentrated feed stocks is discussed based on the gasification kinetic model. (C) 2010 Elsevier B.V. All rights reserved.