The mean activity coefficients of NaBr in the NaBr + Na2SO4 + H2O ternary system were determined at the total ionic strength ranging from (0.0500 to 3.0000) mol.kg(-1) at 298.15 K by potential difference measurements from the galvanic cell without liquid junction: Na-ISE vertical bar NaBr(m(1)), Na2SO4(m(2))vertical bar Br-ISE. The mean activity coefficients of NaBr were compared with those different ionic strength fractions y(B) of Na2SO4 with y(B) = (0.8, 0.6, 0.4, 0.2, and 0). The experimental results showed that Na-ISE and Br-ISE in this work had a good Nernst response, and the mean activity coefficients of NaBr in NaBr + Na2SO4 + H2O mixtures were calculated using the Nernst equation. Mixing interaction parameters of theta(Br-center dot SO42-) and phi(Na+center dot Br-center dot SO42-) in Pitzer's equation, which had not been previously reported, were evaluated from the present measurements of the mean activity coefficients of NaBr. In addition, the osmotic coefficients, water activity, and the excess Gibbs energy of this system were calculated by the Pitzer's equations.