In order to study the hydration of ions in aqueous electrolyte solution, the freezing and successive heating behavior was examined by means of differential thermal analysis. Li+ and Na+ ions as structure-forming ions, and K+ and Cs+ ions as structure-breaking ions, were chosen with NH4+ as an intermediate case. The anion was Cl-. On successive cooling and heating of the solutions, two freezing and melting peaks appeared respectively, except for LiCl solutions. The lower temperature peak (peak I) is considered to be the freezing and melting of the free water, while the higher temperature peak (peak II) corresponds to freezing and melting of the primary and secondary hydration regions : It could be proved that the single peak which appears for LiCl is attributable to the freezing and melting of the free water, since the hydrated region remained unfrozen due to supercooling. From the relative areas of the peaks I and II, the primary plus secondary solvation numbers could be estimated. The dependence of the total solvation number on the salt concentration is considered as due to the change in the secondary solvation number.