Abstract To determine the driving force for calcium sulfate depositions from a supersaturated saline water solution of known composition, three quantities must be calculated: (1) the solubility product defined as [Ca 2+ eq] [SO 2− 4eq], (2) the ratio of the mean activity coefficients of calcium sulfate and (3) the ratio of the activities of water. The solubility product for each calcium sulfate modification was calculated by a modified version of the model derived by Marshall and Slusher. The ratio of the mean activity coefficients of calcium sulfate in the supersaturated solution, and in the saturated solution, as well as the ratio of the activities of water in those solutions were calculated from the ion specific interaction model developed by Pitzer. The consistency of both models was proved by mutual comparison of the solubility curves of gypsum both in sodium chloride solutions and in seawater solutions. Finally an exact calculation of the growth affinity at 25°C was given as well as an adequate approximation, which considerably simplifies the calculation. The same approach is most likely allowed at elevated temperatures.