The thermodynamic properties of NH4Br salt in the ternary NH4Br (m(1)) + NaBr (m(2)) + H2O mixed electrolyte system are reported in this work. A galvanic cell containing a solvent polymeric (PVC) ammonium ionselective membrane electrode (ISE) and an Ag/AgBr electrode was employed for collecting the potentiometric data with a series of mixed salt solutions, at similar constant ionic strengths, and each was characterized by a fixed salt molal ratio r (where r = m(1)(NH4Br)/m(2)(NaBr) = 1, 10, 50, and 100, respectively). Pitzer semiempirical ion-interaction theory for mixed salts was used for modeling the nonideal behavior of the system, over the ionic strength ranging from (0.01 to 5) mol.kg(-1), at 298 K, where the mean activity and osmotic coefficients, solvent activities, and excess Gibbs free energies were determined for this ternary electrolyte system. The impact of the magnitude of the potentiometric selectivity coefficient (K-12(Pot)) of the NH4+ ISE, as a limiting factor, on the results is evaluated.