Decomposition experiments of microcrystalline cellulose were conducted in subcritical and supercritical water (25 MPa, 320-400°C, and 0.05-10.0 s). At 400°C hydrolysis products were mainly obtained, while in 320-350°C water, aqueous decomposition products of glucose were the main products. Kinetic studies of cellulose, cellobiose, and glucose at these conditions showed that below 350°C the cellulose decomposition rate was slower than the glucose and cellobiose decomposition rates, while above 350°C, the cellulose hydrolysis rate drastically increased and became higher than the glucose and cellobiose decomposition rates. Direct observation of the cellulose reaction in high-temperature water at high-pressure conditions by using a diamond anvil cell (DAC) showed that, below 280°C, cellulose particles became gradually smaller with increasing reaction time but, at high temperatures (300-320°C), cellulose particles disappeared with increasing transparency and much more rapidly than expected from the lower temperature results. These results suggest that cellulose hydrolysis at high temperature takes place with dissolution in water. This is probably because of the cleavage of intra- and intermolecular hydrogen linkages in the cellulose crystal. Thus, a homogeneous atmosphere is formed in supercritical water, and this results in the drastic increase of the cellulose decomposition rate above 350°C.