Thermodynamic equilibrium parameters of seven kinds of 1:1 electrolytes of quaternary salts in an organic and aqueous phase, including an extraction constant for ion-pair and free-ion, distribution constant, and dissociation constant in an organic solvent/aqueous solution, were determined by a numerical iteration method. Temperature, the kinds of solvents and quaternary salts, and the concentration of quaternary salts were evaluated to understand the thermodynamic properties. The free energies of extraction constant, distribution constant, dissociation constant, and the transfer of dissociated ions and ion pairs for quaternary ions were obtained as well. The amount of coextracted water and thermodynamic parameters were correlated with the solvent property. The equation of hydrophile-lipophile balance for selecting proper quaternary salt was also developed.