Octakis(2,3,6-tri-O-methyl)-gamma-cyclodextrin (TRIMEG) crystallized from saturated aqueous solution at 18 degrees C in the monoclinic space group P2(1) with 4 TRIMEG and a total of 19.3 water molecules in the asymmetric unit [4(C72H128O40)]. 19.3H(2)O The structure was solved by ab initio real/reciprocal space recycling procedure and refinement converged at an R-factor of 0.084 for 19 782 data with F-2 > 2 sigma(F-2). The structures of the four TRIMEG molecules are similar with O4 atoms nearly coplanar and pseudo-2-fold rotation axes passing through the centers of the macrocycles. The glucoses are oriented syn except for the diametrically opposed glucoses I and 5 which are flipped by similar to 180 degrees (anti), their C5-C6-O6-C9 chains being rotated toward the center and closing the cavity from one side yielding bowl-shaped molecules; the O6 atoms of these chains are hydrogen bonded to water molecules located on the pseudo-2-fold axes. The 19.3 water molecules are distributed over 27 sites hydrogen bonded in different patterns to the 4 TRIMEG.