The mean activity coefficients of KBr in the KBr-K2SO4-H2O ternary system were determined at the total ionic strength ranging from (0.0100 to 3.0000) mol.kg(-1) at 298.15 K by the electromotive force (EMF) method from the battery cell without a liquid junction: K-ISE vertical bar KBr(m(1)), K2SO4(m(2))vertical bar Br-ISE. The mean activity coefficients of KBr were compared with those different ionic strength fractions y(B) of K2SO4 with y(B) = (0.8, 0.6, 0.4, 0.2, and 0). The experimental results showed that K-ISE and Br-ISE in this work had a good Nernst response, and the mean activity coefficients of KBr were fitted to the Pitzer models. Pitzer ion interaction parameters of theta(Br-center dot SO42-) and phi(K+center dot Br-center dot SO42-) which had not been reported were evaluated from present measurements of the mean activity coefficients of KBr.