Phase equilibrium calculation of electrolyte-mixed solvent systems is important in separation processes including extractive distillation. Although methods for equilibrium calculations are available in the literature, they are specific to each application and no general methods have been proposed. In the present work we extented the semiempirical method applicable to the solute properties of mixed electrolyte-mixed solvent systems to the calculation of solvent properties. In the present method excess Gibbs function was represented as the sum of Debye-Hückel term, Born term and NRTL term. Experimental data for mixed 1:1 strong electrolytes-mixed solvent systems showed close agreements with calculated values.