A process model to describe SO2 absorption from exhaust gas using an absorptive magnesium-based slurry was developed in Aspen Plus® V10. The model includes the thermodynamic description of the electrolyte system MgO-CaO-SO2-H2O-O2-CO2, including precipitation reactions in the system. The property method electrolyte-NRTL with an asymmetric reference state was chosen as the thermodynamic method. The model was evaluated using plant data for pH value, HSO3- and SO3-- content of the liquid phase from an industrial SO2 absorption venturi system of the pulp and paper industry. The model shows good accuracy in describing the pH value and the combined HSO3- and SO3-- content. Sensitivity analyses were performed to identify key parameters that influence unwanted precipitation reactions in the system and to support the optimization of the SO2 absorption process. Temperature and the Mg(OH)2/SO2 ratio in the system were identified as key parameters influencing the formation and precipitation of sulfites. The pH value was identified as a key parameter affecting the precipitation of magnesium hydroxide. The model predicts the precipitation of Mg(OH)2 at a pH value of higher than 8 and the precipitation of MgSO3 trihydrate at a temperature higher than 78 °C or a slurry/SO2 ratio higher than around 4. The performed analyses can support optimized process design decisions for SO2 absorption processes to avoid limiting precipitation issues.