The thermodynamic properties of unsymmetrical mixed electrolytes {(y)NH4Cl + (1 - y)(NH4)(2)SO4}(aq) have been measured. The water activities of the mixtures have been measured by the hygrometric method at the total molalities from 0.4 mol(.)kg(-1) to saturation for different ionic-strength fractions of NH4Cl with y = 0.2, 0.5, and 0.8 at the temperature 298.15 K. The results allow the deduction of osmotic coefficients. The experimental results are compared with the predictions of the Pitzer, Zdanovskii-Stokes-Robinson (ZSR), Lietzke and Stoughton (LS II), and Kusik and Meissner (KM) models. From these measurements, the new Pitzer mixing ionic parameters are determined and used to predict the solute activity coefficients in the mixture.