In this work, the thermodynamic properties of the ternary mixed electrolyte system (NaCl + Na2HPO4 + water), using the potentiometric method, are reported. The potentiometric measurements were performed on the galvanic cells without liquid junction of the type: Ag-AgCl vertical bar NaCl (m(1)), Na2HPO4 (m(2)), H2O vertical bar Na-ISE, over total ionic strengths from (0.0130 to 4.000) mol . kg(-1) for different series of salt molal ratios r (r = m(NaCl)/m(Na2HPO4) = 2.5, 5.0, 7.5, and 10.0) and pure NaCl in aqueous system at T = 298.2. The PVC based sodium ion selective electrode (Na-ISE) and Ag-AgCl electrode used in this work were prepared in our laboratory and had a reasonably Nernst response. The activity coefficient results were interpreted based on the Harned rule and Pitzer model. The unknown Pitzer mixing interaction parameters (theta(ClHPO4), psi(NaClHPO4)) were evaluated for the ternary system studied according to Pitzer graphical method. Then, the parameters obtained with the Pitzer model were used to calculate the values of the mean activity coefficients of Na2HPO4, the osmotic coefficients, the solvent activity and the excess Gibbs free energy for the whole series of the studied mixed electrolyte system. (C) 2012 Elsevier Ltd. All rights reserved.