Samples of cellulose II hydrate were prepared by immersion of mercerized cellulose II in anhydrous hydrazine followed by washing with water. The synchrotron X-ray fiber diffraction and solid-state (13)C NMR measurements showed that cellulose II hydrate had a larger unit cell that contained water molecules with lower degree of crystallinity than mercerized cellulose II. but it had basically the same molecular conformation as mercerized cellulose II. The crystal transition from cellulose II hydrate to cellulose II was monitored using synchrotron X-ray diffraction under controlled relative humidity. In the drying process, the cellulose II hydrate gradually contracted, and this decreased the distance between the hydrophobic stacking sheets of molecular chains, while maintaining its sheet structure. The absence of an obvious transition point and the irreversibility of the transition indicated a random location of the water molecules occurred in the unit cell of cellulose II hydrate. On annealing a sample in hot water, the cellulose II hydrate was converted to cellulose II. Cellulose II hydrate was unstable at high temperatures, even in water. (C) 2011 Elsevier Ltd. All rights reserved.