The significant level of crystallinity in atactic poly(vinyl alcohol) results from the comparatively modest size of the OH group enabling it to assume d and I positions in an atactic chain within a single crystal lattice. The structure is highly hydrogen bonded. An X-ray diffraction investigation has been made of the crystal structure to determine the distribution of hydrogen bonds, to understand the crystalline thermal expansion and to determine the influence of water on the polymer crystals and crystallisation process. Thermal annealing treatments have been developed to obtain samples of the maximum level of crystallinity with minimum disorder. The well-resolved diffraction peaks from such samples are analysed to measure the thermal expansion coefficients of the crystal unit cell which are related to the hydrogen bonding network in the crystal structure. Finally, the effect on the crystals of absorbing water into polymer of various levels of crystallinity and also during the crystallisation process is investigated, leading to the hypothesis that water molecules may be incorporated within the random crystal structure resulting in a change in the relative intensities of the diffraction peaks.