The activity of water in the systems Na** plus , NH//4** plus parallel SO//4**2** minus -H//2O and Na**+30,NH//4** plus parallel H//2PO//4** minus -H//2O at 25 degree has been measured by the isopiestic method. Pitzer parameters for these systems have been calculated on the basis of the results obtained. By means of the Pitzer equations, calculations have been made of solubility isotherms for the Na** plus , NH//4 parallel SO//4**2** minus -H//2O and Na** plus , NH//4** plus parallel H//2PO//4** minus -H//2O systems at 25 degree . The solubility data obtained experimentally during the work are in good agreement with the results of calculation.