An analysis of the theory supporting the method to measure the activity of the individual ions using ion-selective-electrodes is presented, and the accuracy of the results obtained is discussed. It is concluded that the effect on the accuracy of the measurements caused by an error in the estimated value of the junction potential, is much less than it was previously thought. Equations are given to correct published data as better information becomes available. The activity coefficients of the bromide ion at 298.2 K, in single electrolyte aqueous solutions of MgBr2 and CaBr2 up to 3 m, and of BaBr2 up to 2.3 m are reported. Direct measurement of the activity of the cations did not produce acceptable results. The activity coefficients of the cations were calculated using values of the mean ionic activity coefficients of the salts. In the presence of the bromide ion the activity coefficient of calcium is only slightly smaller than that of magnesium while the activity coefficient of barium is much smaller than that of calcium. The activity coefficient of the bromide ion is larger in the presence of magnesium, slightly smaller in the presence of barium and smaller in the presence of calcium. The Khoshkbarchi-Vera equation was used to correlate the experimental data. The experimental results for the activity coefficient of individual ions in aqueous solutions are compared with the values predicted by the Hydration Theory of Bates.