The mixture {yNH(4)Cl + (1 - y)MgCl2} (aq) has been studied using the hygrometric method at the temperature 298.15 K. The water activities are measured at total molalities from 0.30 mol kg(-1) up to saturation for different ionic strength fractions y of NH4Cl with y = 0.20, 0.50 and 0.80. The obtained data allow the deduction of osmotic coefficients. Experimental results are compared with the calculations using the models of Zdanovskii-Stokes-Robinson, Kusik and Meissner, Robinson and Stokes, Lietzke and Stoughton, Reilly-Wood and Robinson and Pitzer. Thermodynamic properties have been modeled using the Pitzer ion-interaction model with inclusion of an ionic strength dependence of the third virial coefficient for the binary systems. From these measurements and the obtained binary parameters beta((0)), beta((1)), C-(0) and C-(1), the mixing ionic parameters Theta(NH4Mg), and psi(NH4MgCl) are determined by the standard Pitzer model. The results show that a good accuracy is obtained with the standard Pitzer model using extended binary parameters. The parameters Theta(NH4Mg) and psi(NH4 MgCl) were used for evaluation of activity coefficients in the mixture. The excess Gibbs energy is also determined. (C) 2007 Elsevier B.V. All rights reserved.