Potential difference measurements have been used to obtain the mean ionic activity and osmotic coefficients of NaCl and Na2SO4 in their aqueous solutions. Also, the mean ionic activity coefficients of NaCl in NaCl-Na2SO4-H2O ternary system with sodium as a common ion have been determined from cell potential measurements using: Na-ISE/NaCl (m(NaCl)); Na2SO4 (m(Na2SO4))/Ag/AgCl combined with the reference cell: Na-ISE/NaCl (I = m(NaCl) + 3m(Na2SO4)), AgCl(s)/Ag. The measurements were made at ionic strength ratios for Na2SO4, y(Na2SO4), of (0.0, 0.2, 0.4, 0.6, 0.8, and 1.0), at constant total ionic strengths, I, of (0.1, 0.5, 1.0, 2.0, 3.0, and 4.0) mol.kg(-1) and at temperatures of (298.15, 303.15, and 308.15) K. The experimental mean ionic activity coefficients of NaCl were correlated and analyzed with the Harned rule and Scatchard and Pitzer models. The excess free Gibbs energies of mixing were calculated from the trace activity coefficient data of NaCl and analyzed versus system composition at different temperatures and total ionic strengths.