This paper describes a chemical model that calculates (solid + liquid) equilibria in the {m(1)FeCl(2) + m(2)FeCl(3)}(aq), {m(1)FeSO(4) + m(2)Fe(2)(SO4)(3)}(aq), {m(1)NaCl + m(2)FeCl(3)}(aq), {m(1)NaSO(4) + m(2)FeSO(4)}(aq), {m(1)NaCl + m(2)FeCl(2)}(aq), {m(1)KCl + m(2)FeCl(3)}(aq), {m(1)K(2)SO(4) + m(2)Fe(2)(SO4)(3)}(aq), {m(1)KCl + m(2)FeCl(2)}(aq), {m(1)K(2)SO(4) + m(2)FeSO(4)}(aq), and {m(1)MgCl(2) + m(2)FeCl(2)}(aq) systems, where in denotes molality at T = 298.15 K. The Pitzer ion-interaction model has been used for thermodynamic analysis of the experimental activity data in binary FeCl2(aq) and FeCl3(aq) solutions, and ternary solubility data, presented in the literature. The thermodynamic functions needed (binary and ternary parameters of ionic interaction, thermodynamic solubility products) have been calculated and the theoretical solubility isotherms have been plotted. The mixed solution model parameters {theta(MN) and psi(MNX)} have been chosen on the basis of the compositions of saturated ternary solutions and data on the pure water solubility of the K2SO4 . FeSO4 . 6H(2)O double salt. The standard chemical potentials of four ferrous {FeCl2 .4H(2)O, Na2SO4 . FeSO4 . 4H(2)O, K2SO4 . FeSO4 . 6H(2)O, and MgCl2 . FeCl2 . 8H(2)O} and three ferric {FeCl3 . 6H(2)O, 2KCl . FeCl3 . H2O, and 2K(2)SO(4) . Fe-2(SO4)(3) . 14H(2)O} solid phases have been determined. Comparison of solubility predictions with experimental data not used in model parameterization is given. The component activities of the saturated {m1MgSO4 + m2FeSO4}(aq) and in the mixed crystalline phase were determined and the change of the molar Gibbs free energy of mixing Delta(mix)Gdegrees(m)(s) of crystals was determined as a function of the solid phase composition. It is established that at T = 298.15 K the mixed (Mg,Fe)SO4 . 7H(2)O and (Fe,Mg)SO4 . 7H(2)O crystals show small positive deviations from the ideal mixed crystals. Limitations of the {Fe(II) + Fe(III)} model due to data insufficiencies are discussed. (C) 2004 Elsevier Ltd. All rights reserved.