Abstract: Many chemical and biochemical phenomena take place in solution, making the evaluation of solvent effects a critical element for understanding these phenomena. The effect of the solvent on activity coefficients of electrolytes was of interest to chemists and chemical engineers because it may provide useful information. In this paper the thermodynamic properties of HCl-MgSO4-H2O system have been studied by EMF (electromotive force) measurement in the cells without liquid junction Pt, H2 (101.325 kPa )│HCl (m＝0.01000 mol·kg-1)│AgCl-Ag (A) Pt, H2 (101.325 kPa )│HCl (mA), MgSO4 (mB) , H2O│AgCl-Ag (B) at constant total ionic strength I=0.2, 0.4, 0.6, 0.8, 1.0, 1.5 mol·kg-1 and yB=0, 0.10, 0.20, 0.30, 0.50, 0.70 at different temperatures from 278.15 to 318.15 K. Because of secondary dissociation of sulfuric acid in the system, the concentrations of hydrogen ions in equilibrium system have been determined by mathematic iterative technique. The standard electrode of AgCl-Ag in pure water has been determined according to the method of Bates. The activity coefficients of HCl lgγA in the HCl-MgSO4-H2O system have been determined from cell (B). The results show that the activity coefficients of HCl in the solutions still obey Harned’s rule and lgγA is a linear function of the absolute temperature at constant composition of the mixture.