At a given reaction temperature (175-275 degrees C), the reaction rate constant of glucose decomposition in hot-compressed water (HCW) is found to decrease by almost 1 order of magnitude with increasing the initial glucose concentration from 0.01 to 1000 mg L-1 (equivalent to 5.6 x 10(-8)-0.0056 M). The results demonstrate a clear shift in the glucose decomposition mechanism at various initial glucose concentrations. Fructose is always the dominant reaction product with a high selectivity at the early stage of glucose decomposition, suggesting that it is a key primary product from glucose decomposition in HCW. The initial glucose concentration also influences the selectivity of glucose decomposition products substantially. The hydroxyl ion appears to play a controlling role in catalyzing the isomerization and retro-aldol reactions under all conditions in this study. At an initial glucose concentration less than 10 mg L-1 (i.e., 5.6 x 10(-5) M), particularly when it is close to or less than the molar concentration of the ion product in HCW, the hydroxyl ions play a dominant role in catalyzing the isomerization and retro-aldol condensation reactions to produce fructose, glyceraldehydes and/or glycolaldehyde, erythrose, etc. However, at an initial glucose concentration higher than 10 mg L-1 (i.e., 5.6 x 10(-5) M), the selectivity of 5-hydroxymethylfurfural increases substantially, indicating that hydrogen ions play an enhanced role in catalyzing dehydration reactions under the conditions. The shift in glucose decomposition mechanism is also reflected in the change in apparent activation energy that increases with an increasing initial glucose concentration, mainly due to an increasing selectivity of acid-catalyzed dehydration reaction that has a higher activation energy.