A supercritical ammonia treatment has been used to trap an ammonia-cellulose complex during the conversion of cellulose I to cellulose III1. The crystal and molecular structure of this complex, designated ammonia-cellulose I, has been determined by using X-ray fiber diffraction data (space group P2(1): a = 4.47 angstrom b = 8.81 angstrom, c = 10.34 angstrom, gamma = 92.7 degrees). Although the existence of ammonia-cellulose I has been known for some time, this is the first report of its crystal structure. A one-chain monoclinic unit cell has an asymmetric unit that contains only one glucosyl residue and one ammonia molecule. The ammonia molecule acts as a bridge between hydrogen-bonded sheets, forming extended chains of cooperative hydrogen bonds. The sheets are similar to those found in cellulose III,, with O2...O6 intrasheet hydrogen bonds and the gt conformation of the hydroxymethyl group providing potential bifurcated O3.. O5 and O3... O6 intrachain hydrogen bonds. This new structure provides group providing potential bifurcated O3 a number of insights into the structural transition pathway followed during the conversion of cellulose I to cellulose IIIi.