Densities of solutions of NaCl, CaCl₂ and LaCl₃ in a series of water-methanol mixtures have been measured at 25 ℃．The partial molal volumes of salts V₂^o and the ionic partial molal volumes V^o_ion in water-methanol mixtures have been detemined.
It was found that the applicability of the Debye-Hückel theory decreases in the order NaCl>CaCl₂>LaCl₃ and decreases with increasing methanol content in water-methanol mixtures, and that the electrostriction partial molal volumes of ions V^o_elect are greater in methanol than in water and increase with the valency of the cation. The increments of V^o_elect in water, V^o_elect in methanol, and （V^o_elect in methanol-V^o_elect in water） with the valency of the cation are approximately 25, 55 and 30 ml/mole, respectively.
The results also indicate that up to 0.5∼0.7 mole fraction of methanol （i.e., in water-rich regions of water-methanol mixtures） the preferential hydration around ion occurs, while above this threshold content of water in these mixtures the change of the solvation layer results in the strong electrostriction of the solvent.