In this work, we investigated some thermodynamic properties of the NaCl-CuCl2-H2O ternary system. We designed a cell without liquid junction as follows: Na-ISE vertical bar NaCl (m(1)), CuCl2 (m(2)) vertical bar Cl-ISE, which was used to study the mean activity coefficients of NaCl in the NaCl-CuCl2-H2O system at 298.15 K by the electromotive force method. In this system, the total ionic strength / ranges from 0.01 up to 1.00 mol.kg(-1) for different ionic strength fractions y(b) of CuCl2, i.e., y(b) = (0.0, 0.2, 0.4, 0.6, 0.8). The results show that the Na-ISE and Cl-ISE have a good Nernst effect. The mean activity coefficients of NaCl in the ternary system were calculated using the Nernst equation. The Pitzer ion inter-action parameters theta(Na,Cu), and psi(Na,Cu,Cl) were fitted by Pitzer model with experimental results in this work. In addition, the osmotic coefficients, the solvent activities, and the excess Gibbs free energies of the ternary system were also calculated.