In this investigation, the mixed aqueous electrolyte system of nitrate and sulfate with common ammonium cation has been studied with the hygrometric method at the temperature 298.15 K. The water activities of the system [yNH(4)NO(3) + (1 - y)(NH4)(2)SO4](aq) are measured at total molalities from 0.4 mol kg(-1) up to saturation for different ionic-strength fractions y of NH4NO3 with y = 0.2, 0.5 and 0.8. The obtained data allow the deduction of osmotic coefficients. The experimental results are compared with the predictions of the Zdanovskii-Stokes-Robinson (ZSR), Leitzke and Stoughton (LSH), Kusik and Meissner (KM), and Pitzer models. From these measurements, new Pitzer mixing ionic parameters are determined and used to predict the solute activity coefficients in the mixture. The obtained results are used to calculate the excess Gibbs energy at total molalitics for different ionic-strength fractions y. © 2004 Elsevier B.V. All rights reserved.