The infrared spectra of oriented films of valonia cellulose and of ramie and bacterial cellulose crystallites have been observed in the 3 mu region. Polarization properties of the bands have also been determined. The differences between the polarized spectra of bacterial and ramie crystallites in this region are attributed to per cent crystallinity and orientation effects. Two new bands in the CH stretching region have been observed. With this new information the CH₂ symmetric and antisymmetric stretching modes are assigned to parallel and perpendicular bands, respectively, requiring a specific orientation of the CH₂OH group. From the observed polarization of the bands in the OH stretching region, a system of hydrogen bonding in the crystal structure of cellulose I is proposed. this involves a change in conformation of the cellobiose unit to permit an intramolecular hydrogen bond between the C₃ hydroxyl and the ring oxygen of contiguous glucose units. Two sets of intermolecular hydrogen bonds are proposed: in the 101 plane the C₆ hydroxyls of the antiparallel chains are joined to the bridge oxygens of the adjacent parallel chains; in the 101 plane the C₆ hydroxyls of the parallel chains are hydrogen-bonded to the bridge oxygens of the adjacent antiparallel chains.