Osmotic coefficient data were obtained for the aqueous solutions of NaOH-NaCl-NaAl(OH)(4). The solutions were prepared by dissolving AlCl3 . 6H(2)O in aqueous NaOH solutions. The osmotic coefficients of the solutions were measured by an isopiestic method at 25 degrees C. The osmotic coefficient data were used to evaluate the unknown binary and mixing parameters of Fitter's model for the aqueous NaOH-NaCl-NaAl(OH)(4)-H2O system. The binary Fitter's parameters, beta((0)), beta((1)), and C-phi, for NaAl(OH)(4) were found to be -0.0083, 0.0710, and 0.00184 respectively. These binary parameters were obtained from the data on the ternary system. This was necessary since it was not possible to prepare a single (NaAl(OH)(4)) solution. The mixing parameters, Theta(OH-Al(OH)4-), Theta(Cl-Al(OH)4-), Psi(Na+OH-Al(OH)4-), Psi(Na+Cl-Al(OH)4-) were found to be -0.2255, -0.2430, - 0.0388, and 0.2377 respectively. The expenmentaI osmotic coefficient data were correlated well with Pitzer's model using the parameters obtained.