The water activity and osmotic coefficients of the system {y NH4NO3 + (1-y) KNO3}(aq) has been measured at total molalities from 0.2 mol kg(-1) to about saturation of one of the solutes for different ionic-strength fractions y of NH4NO3 with y = 0.2, 0.5 and 0.8 at the temperature 298.15 K using the hygrometric method. The obtained data allow the deduction of the thermodynamic parameters. From these measurements, new Pitzer ionic mixing parameters are determined and used to predict the solute activity coefficients in the mixture. The results obtained are used to calculate the excess Gibbs energy at total molalities for different ionic-strength fractions of NH4NO3 (C) 2003 Elsevier B.V. All rights reserved.