The water activities and osmotic coefficients of aqueous solutions of {(NH4)(2)SO4 + Li2SO4} and {(NH4)(2)SO4 + Na2SO4} have been determined at a temperature of 298.15 K with a hygrometric method, at molalities in the region 0.2 mol . kg(-1) to saturation of the solutes for different fractional ionic-strengths y = 0.2, 0.5, and 0.8 of (NH4)(2)SO4. The experimental results are compared with the predictions obtained from our extended compared additivity model, as well as the models reported by Zdanovskii, Stokes and Robinson, Pitzer. and Lietzke-Stoughton. Front these measurements, parameters of Pitzers model have been determined, These were used to predict solute activity coefficients in the mixture and calculate the excess Gibbs function at total molalities for different y for these systems.