The chemistry of several calcium sulphate systems was successfully modelled in multi-component acid-containing sulphate solutions using the mixed solvent electrolyte (MSE) model for calculating the mean activity coefficients of the electrolyte species. The modelling involved the fitting of binary mean activity, heat capacity and solubility data, as well as ternary solubility data. The developed model was shown to accurately predict the solubility of calcium sulphate from 25 to 95 degrees C in simulated zinc sulphate processing solutions containing MgSO4, MnSO4, Fe-2(SO4)(3), Na2SO4, (NH4)(2)SO4 and H2SO4. The addition of H2SO4 results in a significant increase in the calcium sulphate solubility compared to that in water. By increasing the acid concentration, gypsum, which is a metastable phase above 40 degrees C, dehydrates to anhydrite, and the conversion results in a decrease in the solubility of calcium sulphate. In ZnSO4-H2SO4 solutions, it was found that increasing MgSO4, Na2SO4, Fe-2(SO4)(3) and (NH4)(2)SO4 concentrations do not have a pronounced effect on the solubility of calcium sulphate. From a practical perspective, the model is valuable tool for assessing calcium sulphate solubilities over abroad temperature range and for dilute to concentrated multi-component solutions. (C) 2007 Elsevier B.V. All rights reserved.