A self-consistent chemical model based on a single set of model parameters for all three CaSO4 modifications-namely, dihydrate, hemihydrate, and anhydrite-was developed and described in this work. The model was successfully tested for the estimation of CaSO4 solubilities in concentrated (up to 20 m) mixed HCI-CaCl2-H2O systems up to 100 degrees C. The new model makes use of the OLI Systems software platform. Via regression of experimental solubility data, new Bromley-Zemaitis model parameters were determined for the Ca2+-SO42-and Ca2+-HSO4- ion pairs. Moreover, for the first time, the new model incorporates data for hemihydrate modification (CaSO(4)centre-dot1/2H(2)O). After validation, the model was calibrated by determining new temperature-dependent parameters of the solubility product constants of the hemihydrate and anhydrite. With the aid of the newly developed model, the bell-shaped solubility curves for dihydrate and anhydrite, as a function of HCl concentration, were successfully explained, based on the bisulfate ion formation and ion-activity coefficient.