Tetradeca-2,6-O-methyl-beta -cyclodextrin, per-2,6-OMe-beta -CD, is an unusual beta -cyclodextrin, beta -CD, derivative because it has a negative thermal solubility coefficient in aqueous solution. This report describes two new crystal structures for hydrates of per-2,6-OMe-P-CD crystallized at different temperatures: per-2,6-OMe-beta -CD . 1.08H(2)O crystallized at 50 degreesC and per-2,6-OMe-beta -CD . 14.7H(2)O crystallized at 4 degreesC. The crystal structure for per-2,6-OMe-P-CD 1.08H(2)O reveals conformational flexibility in the methylated beta -CD molecule; two adjacent glucose residues are disordered as a result of their adopting different tilts with respect to the O-4 plane characterizing the molecule. When one conformation is present there is a water molecule in the lattice (0.5 population parameter); said water is displaced by the methyl of a methoxy substituent when the other conformer is present.