The isopiestic molalities have been measured for the system NaOH-Na₂H₂SiO₄-H₂O by isopiestic method in the ionic strength ranges from 0.5695 to 6.4775 mol•kg^－1 and modulus (mole ratio of SiO₂ to Na₂O) range from 0.1461 to 0.3158 at 313.15 K, and the osmotic coefficients and water activities have been calculated from the measurements. The experimental data have been represented by the Pitzer model of osmotic coefficient. The Pitzer ion interaction parameters were evaluated from experimental data using multiple regression. The experimental data of osmotic coefficients measured and calculated by using the Pitzer model are in reasonable agreements. It is shown that the Pitzer model can describe the systems thermodynamic properties well. The mean activity coefficients of NaOH and Na₂H₂SiO₄ were also calculated for the NaOH-Na₂H₂SiO₄-H₂O system, and these calculations establish their variations with ionic strength and modulus. The mean activity coefficients of NaOH were compared with those calculated from Mckay-Perring equation, and it was pointed out that the application scope of Mckay-Perring equation for NaOH- Na₂H₂SiO₄-H₂O system.

Key words: NaOH-Na₂H₂SiO₄-H₂O system, isopiestic method, osmotic coefficient, Pitzer model, mean activity coefficient, Mckay-Perring equation