Kinetics of hydrothermal reaction of D-glucose was investigated at 0.02 M over a temperature range of 120-160 degrees C by applying in situ C-13 NMR spectroscopy. D-Glucose was found to be reversibly transformed first into D-fructose (intermediate) and successively into 5-hydroxymethylfurfural (S-HMF) through dehydration. The carbon mass balance has been kept within the detection limit, and no other reaction pathways are present. The hydrothermal reaction of D-glucose is thus understood as that of D-fructose in the sense that the D-glucose reaction proceeds only through-D-fructose. All the isomers of D-glucose and D-fructose were detected by the in situ C-13 NMR in D2O: they are the open chains and the pyranoses and furanoses of alpha- and beta-types. The beta-forms are the most stable due to the hydration. For both D-glucose and D-fructose, the isomers are in a rapid equilibrium for each monosaccharide, and they are treated collectively in the kinetic analysis of the slower hydrothermal reactions. The reactions are of the first order with respect to the concentrations of D-glucose and D-fructose, and D-glucose converts to 5-HMF on the order of hours. The kinetic parameters were determined by the in situ method.