Chemical equilibria in the FeSO 4-H 2SO 4-H 2O system have been modelled using the Pitzer formulation for the determination of activity coefficients over the temperature range from 10 to 60°C. The available experimental activity coefficient, enthalpy, heat capacity and mineral solubility data have been analysed to determine the temperature dependencies of the various ion interaction parameters necessary to describe chemical equilibria in this system. In this analysis, the previously published ion interaction parameters in the H 2SO 4-H 2O system have been reevaluated incorporating the higher-order electrostatic terms in the Pitzer formulation to account for asymmetric mixing of ions of dissimilar valence. The combined set of interaction parameters yield an accurate description of equilibria in the FeSO 4-H 2O system over the temperature range of 10 to 90°C and concentration conditions up to the solubility limit of ferrous sulphate hydrate. From an appraisal of solubility data for ferrous sulphate hydrates in water using these parameter expressions, the following relations for the temperature dependence of the solubility products for the heptahydrate (melanterite) and the monohydrate (szomolnokite) phases have been derived: log Ksp = 1.447 - 0.004153T - 214949/T 2Melanterite(FeSO 4·7H 2O) log K sp = 250211 - 0.063405T - 641220/T 2Szomolnokite(FeSO 4·7H 2O) A comprehensive evaluation of ferrous sulphate hydrate solubility data in sulphuric acid solutions enabled the determination of the various additional parameters needed to describe the mixed system, FeSO 4-H 2SO 4-H 2O. The final model yields an excellent representation of all available mineral solubility data for the combined FeSO 4-H 2SO 4-H 2O system over a temperature range from 10 to 60°C, sulphuric acid concentrations from 0 to 6 molal and for iron sulphate concentrations up to the solubility limit.