The water activities of aqueous electrolyte mixture (NaCl + KCl + LiCl + H2O) were experimentally determined at T = 298.15 K by the hygrometric method at total ionic-strength from 0.4 mol (.) kg(-1) to 6 mol (.) kg(-1) for different ionic-strength fractions y of NaCl with y = 1/3, 1/2, and 2/3. The data allow the deduction of new osmotic coefficients. The results obtained were correlated by Pitzer's model and Dinane's mixing rules ECA I and ECA 11 for calculations of the water activity in mixed aqueous electrolytes. A new Dinane-Pitzer model is proposed for the calculation of osmotic coefficients in quaternary aqueous mixtures using the newly ternary and quaternary ionic mixing parameters of this studied system. The solute activity coefficients of component in the mixture are also determined for different ionic-strength fractions y of NaCl. (c) 2006 Elsevier Ltd. All rights reserved.