The mean activity coefficients for NaCl in the (NaCl + SrCl2 + H2O) ternary system were determined by electrode potential measurements of the cell without liquid junction: Na-ion selective electrode (ISE)NaCl (m(A)), SrCl2 (m(B)), H2O vertical bar Cl-ion selective electrode (ISE) at the total ionic strengths ranging from (0.0100 up to 4.0000) mol.kg(-1) at 298.15 K for different ionic strength fractions y(B) of SrCl2 with y(B) = (0.0, 0.2, 0.4, 0.6, and 0.8). The activity coefficient results were interpreted based on the Pitzer model. The Pitzer ion interaction parameters theta(+)(2+)(Na)(center dot Sr) and phi(+)(-)(2+)(Na)(center dot Cl)(center dot Sr) were calculated. Furthermore, those parameters obtained with the Pitzer model were used to calculate the values of the mean activity coefficients of SrCl2. In addition, the osmotic coefficients, solvent activity, and the excess Gibbs free energy for the mixed electrolyte system were also studied.