A model by Cruz and Renon was extended to concentrated mixed strong electrolytes-mixed solvents systems. Calculation was simplified by using solvent volume instead of system volume. Also a systematic method was developed for the determination of parameters applicable to mixed electrolytes-mixed solvents systems. Comparison of calculated activity coefficients of electrolytes and of standard Gibbs free energy of solution with ex-perimental values showed good agreements for NaCl-Methanol-Water, KCl-Methanol-water and NaCl-KCl-Water systems.