Vapor-liquid equilibrium (boiling point) data of the FeSO4-Fe-2(SO4)(3)H2SO4H2O system were measured by the quasi-static ebulliometric method at (30, 60, 90, and 101.3) kPa. The boiling points were found to increase with increasing concentrations of both the salts and the acid. By use of the thermodaynamic data (Delta(0)(Gf,298.15K), Delta H-f,298.15K(0), and S-f,298.15K(0)) of ferric complex ions (FeSO4+, Fe(SO4)(2), and FeHSO42+) predicted by Helgesons equation, a comprehensive thermodynamic model for the system was developed with the mixed-solvent electrolyte (MSE) model. Model parameters were determined via regressing our own experimental boiling points and the literature stoichiometric osmotic coefficients (phi(st)) of the Fe-2(SO4)(3)H2SO4H2O system. The average absolute deviations between the regressed boiling points and the experimental data were found to be (0.30 and 0.36) K for the Fe-2(SO4)(3)H2SO4H2O and FeSO4Fe2(SO4)(3)H2SO4H2O systems, respectively, while the average relative deviation for the stoichiometric osmotic coefficient was only 0.80 %. The model with newly obtained parameters was used to predict the distribution of iron-bearing species (Fe3+, FeSO4+, Fe(SO4)(2), and FeHSO42+) as functions of temperature and iron concentration. In addition, plots of the redox potential of Fe(III)/Fe(II) obtained from the literature versus ln(a(Fe3+)/a(Fe2+)) calculated by the newly obtained parameters are straight lines, indicating the good performance of the parameters.