The Raman and infrared spectra of the hydrates and anhydrides of disodium alpha,omega-alkanedisulfonates NaO3S(CH2)(n)SO3Na (n = 2, 4, 6, and 8) were measured. The TG-DTA curves for the thermal dehydration of the hydrates were obtained. Reversible conformational changes accompanying hydration and dehydration were observed. In the hydrates, the molecules take an all-trans extended form, and the negatively charged SO3- groups of the neighboring molecules are likely to be connected to each other by forming intermolecular hydrogen bonds with the anion water. In the anhydrides, the molecular forms contain the gauche conformation, and the SO3- groups of the neighboring molecules are likely to be separated from each other to reduce intermolecular electrostatic repulsions between the SO3- groups. It is probable that the strain force in the unfavorable gauche conformation competes with this intermolecular electrostatic force in the stabilization of the crystals. The present results show that the anion water plays an important role in the stabilization of the hydrate crystals.