The Electrolyte Nonrandom Two Liquid-Nonrandom Factor (NRTL-NRF) model of Haghtalab and Vera was modified to determine the excess Gibbs free energy of weak electrolyte systems. The model was applied to the phosphoric acid solution as a weak and complex electrolyte. Its thermodynamic behavior was investigated in a wide range of concentration by choosing an appropriate equilibrium reaction between molecular and ionic species. The model contains four adjustable parameters which were determined using osmotic experimental data. The results of dissociation and of activity coefficients are presented separately. (C) 1998 Elsevier Science B.V. All rights reserved.