The partial pressure of ethanol and water in mixtures ranging in composition from 10 to 90% by weight ethanol and containing variable amounts of dissolved salt (NaCI, KCI, or CsCI) to provide a range of salt molalities (based on 1000 g of each mixture) from zero to saturation has been measured. Compilation of these data has been facilitated by vapor chromatographic measurements. The mean molal activity coefficients of these chlorides in the various ethanol-water mixtures have been deduced from these measurements. In the case of NaCI and KCI, the mean molal activity coefficients in the mixed media have also been determined directly in cells without a liquid junction. The cross-differential terms in the Gibbs-Duhem equation have been employed to show that both kinds of measurements (vapor phase chromatographic and potentiometric) yield equivalent results.