The thermodynamic properties of the quaternary aqueous system LiCl + KCl + NH4Cl + H2O have been studied using the hygrometric method by measuring the diameter of droplets of the reference solution of NaCl(aq) as a function of the surrounding relative humidity of the studied system. The water activities of aqueous electrolyte mixtures of LiCl + KCl + NH4Cl + H2O were experimentally determined at T 298.15 K at a total ionic strength from 0.6 mol.kg(-1) to 4.8 mol.kg(-1) for different ionic strength fractions y of one of the three solutes (y = I-XCl/I-tot), ranging from 1/3, 1/2, to 2/3 with the constant ratio r of molality of the two other solutes of unity (r = m(YCl)/m(ZCl)). The data allow the deduction of osmotic coefficients. The results obtained were correlated using Pitzer's model, Dinane-Pitzer's model, the Robinson-Stokes (RS) equation, the Lietzke-Stoughton (LS II) model, and Dinane's mixing rules ECA I and ECA II. The solute activity coefficients of the components in the mixture were also determined for different ionic strength fractions y of solutes.