The degradation kinetics of the monosaccharides of both the aldohexose (glucose, galactose and mannose) and ketohexose (fructose and sorbose) types in subcritical water were studied using a tubular reactor in the temperature range of 180-260 degreesC and at a constant pressure of 10 MPa. The relationship between the fraction of unreacted monosaccharide and the residence time could be expressed by the Weibull equation for all the monosaccharides at any temperature. The susceptibility to degradation was quite different among the saccharides. The activation energy and frequency factor were calculated from the Arrhenius equation and the enthalpy-entropy compensation was found to hold for the degradation of monosaccharides in subcritical water. The isokinetic temperature was similar to that for the hydrolysis of disaccharide. The conversion to 5-hydroxymethy-2-furaidehyde (HMF) and the decrease in pH of the effluent solution were measured and it was found that fructose had the highest value for both. (C) 2004 Elsevier Ltd. All rights reserved.