In this research, thermodynamic properties of the ternary electrolyte system (MgCl2 + Mg(NO3)(2)+ H2O) were investigated using a potentiometric method. The galvanic cell used had no liquid junction of type: Mg-ISE vertical bar MgCl2 (m(A)), Mg(NO3)(2) (m(B)), H2O vertical bar Ag/AgCl. The measurements were performed at T= 298.15K and at total ionic strengths from 0.001 to 8.000 mol/kg for different series of salt ratios r = m(Mgcl2)/m(Mg(NO3)2) =1.00, 2.50, 5.00, 7.50, 10.00 and 15.00. The PVC based magnesium ion-selective electrode (Mg-ISE) and the Ag/AgCl electrode used in this work were prepared in our laboratory and showed a reasonably good Nernst response. The Pitzer ion interaction model and Harned rule were used to illustrate the ternary electrolyte system investigated. The experimental results showed that both Pitzer model and Harned rule were suitable to be used satisfactorily to describe this ternary system. (C) 2010 Elsevier B.V. All rights reserved.