We have developed a comprehensive thermodynamic model for the ternary system sulfur dioxide + sulfuric acid + water based on a previously published thermodynamic model of the aqueous sulfuric acid system using the symmetric electrolyte NRTL (eNRTL) activity coefficient model. The eNRTL binary interaction parameters and the chemical equilibrium constants are regressed from experimental SO2 solubility data in aqueous sulfuric acid solutions. The model accurately represents all thermodynamic properties including vapor-liquid equilibrium, liquid-liquid equilibrium, calorimetric properties, and speciation over a wide acid concentration range, from pure water to pure sulfuric acid and pure sulfur dioxide, and temperatures from 273.15 to 393.15 K. The model should be very useful in supporting process research, development, and design of advanced water-splitting processes based on the hybrid sulfur (HyS) cycle. (C) 2017 Elsevier B.V. All rights reserved.