Ion-selective electrodes (ISEs) were used to measure the activity coefficients at 298.15 K of individual ions in aqueous solutions of NaCl, NaBr, KCl and NH4Cl up to 3m. The possible effect of the standard solution used in the reference electrode was tested. The emf responses of ISEs were measured against a single-junction reference electrode filled with either a 4 M KCl Solution, saturated with AgCl, or a 2 M NH4Cl solution. For the first three systems, ISEs for both the cation and the anion were used. For NH4Cl, only the response of a chloride ISE was measured. The number of experimental points measured in the dilute region was increased and the effect of the maximum composition used to determine electrode parameters was studied. It was found that the reference solution used for the measurements has a minor effect on the measured values of the individual ionic activities while the maximum sample composition to which the normalization condition is applied has a larger effect. The ionic activity coefficients may fail to satisfy the necessary condition of reproducing the mean ionic activity coefficient if the maximum normalization concentration exceeds 0.01m. Otherwise, values of the ionic activities reproduce well literature values for the mean ionic activity coefficients. (c) 2005 Elsevier B.V. All rights reserved.