The osmotic coefficients of aqueous mixtures of potassium chloride and sodium chloride were evaluated at 45 degrees C by the isopiestic method, varying the total molal concentration between 1 and 5 moles kg(-1) in the whole range of reduced molar fraction. The results obtained were correlated by the Pitzer equation and by that derived by Pan and Han, for the application of the McKay-Perring method. From these correlations, the activity coefficients of both salts in the mixture were calculated. A comparative analysis of the results obtained were also made.