The solubility of MgSO4 center dot 7H(2)O in the H2SO4-H2O and Ti(SO4)(2)H2SO4-H2O systems was investigated from (278.2 to 308.2) K with initial H2504 concentrations of (0 to 5.5) mol-kg-1 by dynamic method. It was found that the solubility of MgSO4 center dot 7H(2)O in the H2SO4-H2O system gradually decreased and then slightly increased with the concentration of 112504. Each solubility curve has its own minimum which is temperature dependent. For the MgSO4-Ti(SO4)(2)-H2SO4-H2O system, the solubility of MgSO4 center dot 7H(2)Odecreased sharply with increasing concentrations of H2SO4 and Ti(SO4)(2) throughout the entire range investigated. A new chemical model was developed by regression of the experimental data for MgSO4 center dot 7H(2)O solubility to obtain new middle range interaction parameters for the mixed -solvent electrolyte (MSE) model. The model, with the newly obtained parameters, was applied to predict the speciation, water activity, and ionic activity coefficients in the MgSO4-H2SO4-H2O and MgSO4-Ti(SO4)(2)-H2SO4-H2O systems.