The water activities of aqueous electrolyte mixture NH4Cl-CaCl2-H2O were experimentally determined at T = 298.15 K by the hygrometric method at total ionic-strength from (0.6 to 9) mol . kg(-1) for different ionic-strength fractions y of NH4Cl 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 our proposed mixing rule ECA (extended composed additivity) for calculations of water activity in mixed aqueous electrolytes. From these measurements, the Pitzer mixing ionic parameters are estimated and used to predict the solute activity coefficients in the mixture. (C) 2004 Elsevier Ltd. All rights reserved.