This work reports the results of the thermodynamic modeling of NH(4)Cl salt in the ternary NH(4)Cl (m(1)) + NH(4)NO(3) (m(2)) + H(2)O mixed electrolyte system based on potentiometric measurements. The electrochemical measurements were performed, at similar constant ionic strengths, in different series of mixed salt solutions, each characterized by a fixed salt molal ratio r (where r = m(1)/m(2) = 0.05, 1, 5, 10, and m(1,) m(2) are the molalities of NH(4)Cl and NH(4)NO(3), respectively). The galvanic cell used for collecting the potentiometric data consisted of a solvent polymeric (PVC) NH(4)(+) selective membrane electrode (NH4+ ISE) and an Ag/AgCl electrode. Pitzer ion-interaction equations for mixed salts were used for modeling the nonideal behavior of the system, over the ionic strength ranging from 0.01 mol center dot kg(-1) up to about 6 mol center dot kg(-1), at 298.15 K. On the basis of the obtained Pitzer ion-interaction parameters, the osmotic coefficients, solvent activities, and excess Gibbs free energies were also determined for these investigated ternary electrolyte systems.