Water activities have been measured for the system {yNH(4)Cl+ (1 -y)NH4NO3 +H2O}, at total electrolyte molalities from 0.2mol kg(-1) to about saturation for ionic-strength fractions of NH4Cl y=0.20, 0.50 and 0.80, at temperature 298.15K, by the hygrometric method. The obtained data allow the deduction of osmotic coefficients. The experimental results are compared with the Leitzke and Stoughton rule, and Kusik and Meissner, Robinson and Stokes and Pitzer models. Good accuracy is obtained by Pitzer's model. The use of Pitzer's model involves the adjustment of the parameters theta and psi. The other models do not require parameter fitting. From these measurements, 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 molalities for ionic-strength fractions y. (c) 2005 Elsevier B.V. All rights reserved.