The crystal and molecular structure, together with the hydrogen-bonding system in ammonia-mercerized cellulose IIII has been determined using synchrotron X-ray and neutron fiber diffraction data. The structure has a one-chain monoclinic unit cell with an asymmetric unit that contains only one glucosyl residue and with the hydroxymethyl group in the gt conformation. The hydrogen-bonding system is well-defined with no evidence of disorder. A bifurcated hydrogen bond links a donating secondary alcohol 03 atom to a ring 05 atom (major) and a primary alcohol 06 atom (minor) of an adjacent residue in the same chain. Two hydrogen bonds are present between neighboring chains, perpendicular to the chain direction. A detailed comparison of the crystal structure and hydrogen-bonding system reported here for cellulose IIII and those reported previously for the other cellulose polymorphs is given. The conformation of the chain in cellulose IIII, has features similar to that of the center chain in the highly cellulose II allomorph. However, unlike in cellulose II, the chains are parallel, as in cellulose I-alpha, and cellulose I-beta.