The crystal structure of beta-cyclodextrin hydrate was determined for crystals at four ambient humidities between 15% and 78% and for a crystal in contact with mother liquor. In this humidity range, fast, continuous, and reversible dehydration is observed, which does not affect the ordered crystalline structure. At very low humidities, the crystalline structure irreversibly collapses due to the loss of water. The water content per beta-cyclodextrin molecule gradually reduces from 12.3 H2O at 100% to about 9.4 H2O at 15% humidity. The dehydration occurs according to a simple law e(-kt), and equilibration is reached within a few minutes. The crystal packing arrangement, the pattern of intermolecular hydrogen bonds, and the conformation of the beta-cyclodextrin molecules are conserved during dehydration. The loss of crystal water is due to continuous change of the occupanices of partially populated water sites; this affects the water molecules included in the beta-cyclodextrin cavity to a larger extent than the interstitial water molecules. These observations imply fast diffusion of water molecules in the solid, although no permanent continuous channels are present in the crystal structure to permit easy exchange of water molecules. The mechanism of water diffusion must therefore involve positional fluctuations of the beta-cyclodextrin atoms.