The electrolyte-NRTL model was applied for the representation of VLE of the system H3PO4-H2O. The thermodynamic behavior of this system was investigated in wide range of temperature and concentration, under the operating conditions relating to the manufacturing of phosphoric acid and of the phosphate fertilizers. The liquid phase composition was estimated by choosing two mechanisms (i and ii): (i) assumes that only the first dissociation of H3PO4 takes place and (ii) considers in addition of the later chemical equilibrium, the reaction formation of H5P2O8-. At 298.15 K and with the assumption of equality of activity coefficients of anions (f(H2PO4)(-) f(H5P2O8)(-)) for mechanism (ii), six interaction parameters for each mechanisms were estimated from osmotic coefficient and water activity data. At the elevated temperatures, only the interaction parameters between H3PO4 and H2O for the first mechanism were determined as function of temperature from the vapor pressure data. The results of each mechanisms were presented separately and were compared with the experimental data and with the previous works. (c) 2005 Elsevier B.V. All rights reserved.