Thermodynamic models for aqueous Ba2+ -SO42--Na+-Cl--solutions are compared in their accuracy to predict ion activities in saturated and supersaturated solutions. The Pitzer and the Bromley model are employed, taking into account ion pair formation of barium sulfate. Such models are then used to describe particle nucleation and growth, and finally they are imbedded in a mechanistic mixing-precipitation model for a single feed sernibatch process. The effect of the key operating parameters on the mean particle size is analyzed through simulations. The results are compared with previous experimental data, thus highlighting the significance of a proper choice of the thermodynamic model.