Abstract: Solubility data of KClO₄ in mixed solvents, namely, methanol-benzene, methanol-toluene, methanol-Carbontetrachloride and methanol-cyclohexane, are reported in this paper, The "free ion" concentrations of KClO₄ and KNO₃ are calculated by using the association constants given by A,D'Aprano and C. W. Davies[2] of these salts in methanol, The medium activity coefficients fMXexpressed in terms of S₀/S_m and (S₀)_±/(S_m)_± are approximately equal and related to rnol fraction X_Nc of nonelectrolytes in the mixed solvents by the following empirical formula: lgf_MX = kX_Ne where So and Sm are solubilities of salts in methanol and mixed solvensts respectively; the Subscript ± is used for the "free ion" and k is a constant. The formula for the Solvation number of electrolyte n₊ +n_-=-2lgf_MX/lgφ_p is not valid for KClO₄ in these Mixed solvents, where n₊ +n_- is the sum of the solvation number of positive and negative ion; φ_p is the volume fraction of methanol in mixed solvents, This result indicates that the solvation layer of ClO₄^- is not entirely formed by the methanol molecules and the nonelectrolyte exerts some effects on the solvation layer too.experimental data of KNO₃, formulas of the solubility and of Solvation numbers are quoted from the former paper of li, Huang and liu^[1].