A theoretical and experimental investigation of solubility of gypsum and anhydrite IT in the complex system CaSO4 + MgSO4 + H2SO4 + H2O was conducted from 298.1 to 363.1 K. The solubility of anhydrite II in (MgSO4 + H2SO4) aqueous solutions was determined at 348.1 and 363.1 K by an isothermal method. A Pitzer thermodynamic model was selected to predict the solubility of gypsum and anhydrite II in the titled quaternary system. Good agreement between the experimental and the model values was observed, which supports the reliability of the model prediction. As predicted by the model, the transfer temperature from gypsum to anhydrite II decreases with increasing H2SO4 and MgSO4 concentrations, and the solubilities of gypsum and anhydrite II decrease monotonically with increasing MgSO4 concentration if H2SO4 concentration is higher than 0.5 mol.kg(-1). On the basis of the results obtained, several suggestions for avoiding calcium sulfate scaling in the industrial process were proposed.