A phase equilibrium model of the O-2-ZnSO4-H2SO4-H2O system with the mixed solvent electrolyte (MSE) model is presented. The available thermodynamic properties, including mean activity coefficient of ZnSO4 in water, and the solubility of ZnSO4 in water and in H2SO4 solutions were modeled first to obtain model parameters: equilibrium constants for solids and the MSE ion interaction parameters between Zn2+ -SO42-and Zn2+ -HSO4- ion pairs. The solubility of O-2 in the ZnSO4-H2SO4-H2O system was modeled with additional interaction parameters between O-2(aq) and ions (H+, HSO4-, and Zn2+). The application of the MSE thermodynamic frameworkto O-2-ZnSO4-H2SO4-H2O shows that the model is able to give an accurate prediction of different types of experimental data in binary (ZnSO4-H2O), ternary (ZnSO4-H2SO4-H2O), and quaternary (O-2-ZnSO4-H2SO4-H2O) systems. (c) 2005 Published by Elsevier B.V.