Cellulose II hydrate was prepared from microcrystalline cellulose (cellulose I) via its mercerization with 5 N NaOH solution over 1 h at room temperature followed by washing with water. The structure of cellulose II hydrate changed to that of cellulose II after drying. Compared with cellulose II, cellulose II hydrate exhibited a slightly (8.5%) expanded structure only along the 1 1 ¯ 0 direction. The hydrophobic stacking sheets of the cellulose II were conserved in the cellulose II hydrate, and water molecules could be incorporated in the inflated two-chain unit cell of cellulose II hydrate. Enzymatic hydrolysis of cellulose I, cellulose II hydrate, and cellulose II was carried out at 37 °C using solutions comprising a mixture of cellulase and β-glucosidase. The hydrolysis of cellulose II hydrate proceeded much faster than the hydrolysis of the other two substrates, while the saccharification ratio of cellulose II was only slightly higher than that of cellulose I. The alkaline mercerization treatment was also applied to sugarcane bagasse. After its direct mercerization, the cellulose in bagasse was converted from cellulose I to cellulose II hydrate, and then to cellulose II after drying. Similar to in the case of microcrystalline cellulose, the rate of the enzymatic hydrolysis of the mercerized bagasse without drying (cellulose II hydrate) was much faster than the enzymatic hydrolysis of the other two substrates. Thus, the wet forms of cellulose and cellulosic biomass after mercerization, and after hydrolysis with cellulolytic enzymes, afforded superior products with extremely high degradability.