The mean activity coefficients for NaBr in the (NaBr + SrBr2 + H2O) ternary system were determined by electromotive force (EMF) measurements of the following cell without liquid junction: Na+ISENaBr (m(A)), SrBr2 (m(B))BrISE At the total ionic strengths ranging from 0.01 mol.kg(1) up to 2.00 mol.kg(1) at 298.15 K for different ionic strength fractions y(B) of SrBr2 with y(B) = (0.0, 0.2, 0.4, 0.6, and 0.8). The activity coefficient results were interpreted based on the Harned rule and Pitzer model. Furthermore, the Pitzer ion interaction parameters theta(Na+Sr2+) and theta(Na+Br-Sr2+) were calculated. those parameters obtained with the Pitzer model were used to calculate the values of the mean activity coefficients of SrBr2, the osmotic coefficients, solvent activity, and the excess Gibbs free energy for the whole series of the mixed electrolyte system that was studied.