The solubilities and the refractive indices of the MgBr2-MgSO4-H2O system at 333.15 K were studied with the isothermal equilibrium solubility method. The phase diagram and refractive index diagram were plotted for this system at 333.15 K. The system belongs to hydrate II type. The phase diagram is constituted of three invariant points cosaturated with two salts, four univariant solubility isotherms saturated with one salt, and four stable crystallization fields in the ternary system corresponding to MgSO4 center dot 6H(2)O (Hex), MgSO4 center dot 4H(2)O (Tet), MgSO4 center dot H2O (Kie), and MgBr2 center dot 6H(2)O (Mb). The calculated refractive index data agree well with the experimental results. Combining the experimental solubility data of the ternary system, the Pitzer binary parameters for MgBr2 and MgSO4, and the Pitzer mixing ion-interaction parameter theta(Br,SO4), the Pitzer parameter psi(Mg,Br,SO4) and the solubility equilibrium constants K-sp of solid phases in the ternary system at 333.15 K were fitted using the Pitzer and Harvie-Weare (HW) model. Then the solubilities for the ternary system at 333.15 K were calculated. A comparison between the experimental and calculated solubilities illustrates that the predicted data obtained with the model are in accordance with experimental results.