The component solubilities of FeSO4·7H2O–ZnSO4·xH2O–H2O (x = 6, 7) systems at 25 and 40°C were calculated using Pitzer’s ion-interaction model and its Harvie–Weare extension. The calculated results of FeSO4 · 7H2O–ZnSO4· xH2O–H2O (x = 6, 7) systems are in good agreement with the experimental data both at 25 and 40°C. The model can be extended to a wider range of temperatures for single- or multi-component systems using temperature-dependent binary parameters, providing an alternative approach for the prediction of electrolyte solubilities.