Vapor-liquid equilibrium data (water activity, vapor pressure, osmotic coefficient, and activity coefficient) of the mixed electrolyte aqueous solution, 1-ethyl-3-methylimidazolium bromide, [Emim][Br], + tripotassium phosphate, [Emim][Br], + dipotassium hydrogen phosphate, [Emim][Br], + potassium dihydrogen phosphate, and their corresponding binary aqueous solutions have been measured by the isopiestic method at temperature 298.15 K. The osmotic coefficients for binary aqueous solutions were correlated to the Pitzer and modified Pitzer models. From these data, the corresponding mean molal activity coefficients, gamma(+/-), have been calculated. The activity coefficients of mixed electrolytes were calculated by Scatchard's neutral-electrolyte method. The activity results were also satisfactorily fitted to the semiempirical equation.