Osmotic coefficient and water activity data for Na2SiO3 and mixed Na2SiO3-NaOH aqueous solutions were obtained at 25 degrees C by employing the isopiestic method. The binary Fitter's parameters, beta((0)), beta((1)), and C-phi, for Na2SiO3 and the mixing parameters, theta(OH-SiO32-) and Psi(Na+OH-SiO32-) were estimated using the osmotic coefficient data. The experimental osmotic coefficient data were correlated well with Fitter's model. Mean activity coefficients of Na2SiO3 and NaOH for the solutions were predicted by Fitter's model with the parameters obtained. Finally, the presence of metasilicic species, when sodium metasilicate (Na2SiO3. 9H(2)O) is dissolved in aqueous alkaline solution, was established by titration with hydrochloric acid. (C) 1998 Elsevier Science B.V. All rights reserved.