The mean activity coefficients for NaCl in a ternary electrolyte system were determined by the potentiometric method, at 25 degrees C, using a solvent polymeric (PVC) sodium-selective membrane electrode (Na+ ISE), containing N,N-dibenzyl-N,N'-diphenyl-1,2-phenylenedioxydiacetamide as ionophore, and combined with an Ag/AgCl electrode. The potentiometric measurements were performed at the same ionic strengths in different series of mixed salt solutions, each characterized by a fixed salt molal ratio r (where r = m(1)/m(2) = 1, 10, 50, 100). The nonideal behavior of the ternary NaCl(m(1)) + LiCl(M-2) + H2O electrolyte system was described based on the Pitzer ion-interaction model for mixed salts over the ionic strength ranging from 0. 01 up to about 4 mol/kg. Two-and three-particle Pitzer interaction parameters for a mixed electrolyte system were determined based on potentiometric data, and the critical role of potentiometric selectivity coefficient (K-12) of ISE as limiting factor in the potentiometric measurements was analyzed.