The thermodynamic properties of the quaternary aqueous system NaCl-NH4Cl-LiCl-H2O have been studied using the hygrometric method. The water activities of this aqueous electrolyte system were experimentally determined at T = 298.15 K at total ionic-strength from 0.4 to 6 mol kg(-1) for different ionic-strength fractions y of one of the three solutes ranging from 1/3 to 2/3 with the constant ratio r of molality of the two other solutes of one. The experiments were performed by measuring the diameter of droplets of the solutions as a function of the surrounding relative humidity. The data allow the deduction of osmotic coefficients. The results obtained were correlated by the Pitzer model, the Dinane-Pitzer model, the Robinson-Stokes (RS) equation, the Lietzke-Stoughton (LSII) model, and the Dinane mixing rules ECA I and ECA II. The solute activity coefficients of components in the mixture are also determined for different ionic-strength fractions y of solutes. (C) 2008 Elsevier B.V. All rights reserved.