The solubilities of FeSO4 center dot 7H(2)O in the H2SO4Ti(SO4)(2)H2O and H2SO4MgSO4H2O systems were measured with a dynamic method from 278.15 to 308.15 K. Due to the hydrolysis property of Ti(IV), it was found that the common ion effect of Ti(SO4)(2) was not so obvious as that of MgSO4 for the solubility of FeSO4 center dot 7H(2)O. The solubility of FeSO4 center dot 7H(2)O in the HClH2O system was also measured by same method. The solubility did not change much at first, yet substantially increased when more HCl was added into the solution. New interaction parameters of ions for the mixed-solvent electrolyte (MSE) model were regressed by the experimental solubility data, with the average relative deviations of less than 5.0%. The speciation and the activity coefficients of species in the FeSO4-HCl-H2O system with an FeSO4 concentration of 1.0 mol.kg(1) were predicted by the MSE model with the newly regressed parameters at 303.15 K.