This work reports the results of thermodynamic investigation of the ternary mixed-electrolyte system (MgCl2 + MgSO4 + H2O). The investigation was performed based on Pitzer ion interaction model by using the potentiometric technique at T = 298.15 K. The mean activity coefficients of MgCl2 in the mixed aqueous electrolyte system were determined on the galvanic cell without liquid junction of the type: Mg-vertical bar SE vertical bar MgCl2 (m(A)), MgSO4 (m(B)), H2O vertical bar Ag/AgCl over total ionic strengths from (0.001 to 8.000) mol . kg(-1) for different series of salt ratio r (r = m(MgCl2)/m(MgSO4) = 2.5, 5.0, 7.5, 10.0, 15.0 and pure MgCl2). The PVC based magnesium ion-selective electrode (Mg-ISE) and the Ag/AgCl electrode used in this work were made in our laboratory and had a reasonably good Nernst response. The experimental results obeyed the Harned rule, and the Pitzer model could be used to describe this ternary system satisfactorily. Pitzer ion-interaction parameters for mixed salts were determined for this system. Then, these parameters (0(CISO4) = 0.0252 +/- 0.0042, psi(MgClSO04) = 0.0049 +/- 0.0003) were used to calculate the values of the mean activity coefficients of MgSO4, the osmotic coefficients of water (phi) and the excess Gibbs free energies of solution (G(E)) for the whole series of the studied electrolyte systems. (C) 2011 Elsevier Ltd. All rights reserved.