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[en] Highlights: • Ion interaction (Pitzer) model of aqueous ZnSO4 solutions from 268 K to 373 K. • Critical evaluation of decomposition relative humidities of ZnSO4 hydrates. • Calculation of solubilities of ZnSO4 hydrates. • Complete phase diagram of ZnSO4–H2O system. - Abstract: This paper reports a comprehensive thermodynamic treatment of the binary system ZnSO4–H2O including solution properties and phase equilibria involving various hydrates of zinc sulfate. Published thermodynamic data of ZnSO4(aq), solubilities of the hydrates of zinc sulfate and equilibrium data of the hydration–dehydration reactions have been collected and critically assessed. The more reliable solution properties were used to evaluate the parameters of an extended form of Pitzer’s ion interaction model. With this model the activity coefficients and water activities of saturated ZnSO4 solutions have been calculated yielding, together with equilibrium data of hydration–dehydration reactions, the thermodynamic solubility products of the three stable phases, ZnSO4·7H2O (goslarite), ZnSO4·6H2O (bianchite), ZnSO4·H2O (gunningite), and the metastable, monoclinic ZnSO4·7H2O. Using these solubility products together with the ion interaction model, the predicted solubilities are in excellent agreement with the experimental data. The complete phase diagram of the system has been established in the temperature range (266–378) K.