beta-Cyclodextrin (beta-CD) crystallizes from 27% DMSO-water as beta-CD(.)0.5DMSO(.)7.35H(2)O in the monoclinic space group P2(1) with unit cell constants: a = 15.155(1), b = 10.285(1), c = 20.906(1) Angstrom, beta = 109.86(1)degrees. Anisotropic refinement of 888 atomic parameters against 9,127 X-ray diffraction data converged at an R-factor of 0.055. The beta-CD macrocycle adopts a 'round' conformation stabilized by intramolecular, interglucose O-3(n)... O-2(n + 1) hydrogen bonds. In the beta-CD cavity, DMSO, water sites W-1, W-3 (occupancies 0.5, 0.25, 0.75) are not located concurrently with the water site W-2 because the interatomic distances to W-2 are too short (1.56-1.75 Angstrom). DMSO is placed in the beta-CD cavity such that its S-atom is shifted from the O-4 plane center to the beta-CD O-6-side ca. 0.9 Angstrom and the C-S bond which is inclined 13.6degrees to the beta-CD molecular axis. It is maintained in position by hydrogen bonding to water site W-3 and the O-31-H group. 7.35 water molecules are extensively disordered in 13 positions both inside (W-1-W-4) and outside (W-5-W-13) the beta-CD cavity. They act as hydrogen bonding mediators contributing significantly to the stability of the crystal structure. (C) 2002 Elsevier Science Ltd. All rights reserved.