The mixed aqueous electrolyte system with ammonium and sodium nitrate {NH4NO3 + (1 - y)NaNO3}-(aq) has been studied by the hygrometric method at a temperature of 298.15 K. The water activity of this system has been measured at total molalities from 0.4 mol center dot kg(-1) to about saturation for different ionic-strength fractions, y, of NH4NO3 of 0.20, 0.50, and 0.80. The experimental results are compared with the Zdanovskii-Stokes-Robinson (ZSR), Lietzke and Stoughton (LSII), Kusik and Meissner (KM), and Pitzer models. From these measurements, the Pitzer mixing ionic parameters were determined and used to calculate the solute activity coefficients in the mixture. The obtained results were used to calculate the excess Gibbs energy.