Individual Na+ and Cl- ionic activity coefficients were measured in aqueous solutions of NaCl at 298.15 K in a homoionic system. The emf of sodium and chloride half-cell ion-selective electrodes (ISEs) was measured against a single junction reference electrode of Ag/AgCl, filled with NaCl solutions. Experimental runs were performed at concentrations 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 m of the filling solution. In each run, the concentration of the sample solution was increased from around 1 x 10(-3) m, up to the corresponding molality of the filling solution. Thus, the last point of each run had a zero liquid-junction potential. The ionic activity coefficients measured in this work were found to be independent of the molal concentration of the filling solution used and in good agreement with previous results obtained using heteroionic systems. (c) 2007 Elsevier Ltd. All fights reserved.