The usual approach in modeling electrolyte systems consists in using only VLE experimental data or derived properties values to adjust the thermodynamic model parameters. In the present work it has been showed that this approach, when applied to H3PO4-HO and H2SO4-H2O systems, fails to represent the real composition of the liquid phase with a good accuracy. In this paper as it was pointed out in our previous work [1], we present the result of a new modeling approach based on a large experimental database including Raman spectroscopic liquid phase composition. Both the excess properties (osmotic coefficients) and the speciation in terms of H+. HSO4- and SO42- have satisfactorily been modeled with only six parameters of the Pitzer's model. Furthermore, several chemical equilibria involving ionic and/or molecular species have been taken into account and calculations were led by solving the problem formulated as Gibbs energy minimization. Our results compare well with experimental data within a large domain of acid concentration.