Single ion activity coeffients of NaCl, KCl, HCl and CaCl2 in aqueous solution have been estimated by means of ion selective electrode (ISE) potentiometric measurements. Two methods are described for the calibration of the electrodes within the extended Debye-Hückel concentration range, using the Henderson-Bates approximation for the diffusion potential arising at the reference electrode. The consistency of the results indicates that the junction potentials in the examined systems calculated by the Henderson-Bates approximation are of reasonable precision. The published methods and data might be useful to develop single ion parameters for individual ion activity coefficient models.