A thermodynamics-based model for sodium aluminosilicate formation in aqueous alkaline solutions was developed. Fitter＇s method was adopted to calculate the activity of water and the activity coefficients of the other species in solution. The system under consideration contained the ions of Na+, Al(OH)(4)(-), SiO32-, OH-, CO32-, SO42-, Cl-, and HS- dissolved in water and in equilibrium with two possible solid phases (sodalite dihydrate, Na-8(AlSiO4)(6)Cl-2. H2O; hydroxysodalite dihydrate, Na-8(AlSiO4)(6)(OH)(2). 2H(2)O) at 368.15 K. The equilibrium constants of sodalite dihydrate and hydroxysodalite dihydrate formation reactions were determined using the thermodynamic properties of the species involved. Property values that were not available in the literature were estimated by group contribution methods. The model calculates the molality of all species at equilibrium including the amount of solid precipitates. The results were found to be sensitive to the value of the equilibrium constant for hydroxysodalite dihydrate formation and generally in good agreement with experimental values.