Single ion activity coefficients of H+ and Br- in aqueous hydrobromic acid solutions, gamma(H+) and gamma(Br-), have been estimated potentiometrically at 25 degrees C with a hydrogen electrode and a silver-silver bromide electrode, respectively, based on the assumption that the change in the liquid junction potential at the interface between a sample solution and an ionic liquid salt bridge is negligibly small. The mean activity coefficients of HBr calculated from experimentally obtained values of gamma(H+) and gamma(Br-) well agree with the literature values over the entire concentration ranges studied, 1.04 x 10(-3) to 0.528 mol kg(-1). Overall dependencies of gamma(H+) and gamma(Br-) on ionic strength are similar to those found previously in aqueous HCl solutions, but the degree of the dependence is distinctively stronger in both gamma(H+) and gamma(Br-). Plots of gamma(H+) and gamma(Br-) a function of the ionic strength are nearly congruent with the predictions by Fraenkel's smaller-ion shell (SiS) model up to 0.05 mol kg(-1). However, with a further increase in HBr concentration experimental gamma(H+) and gamma(Br-) more rapidly increases and decreases, respectively, in comparison with the SiS predictions. (C) 2013 Elsevier Ltd. All rights reserved.