Electromotive force (EMF) data were measured at 298.15 K for the cell, Na-ISE \ polyelectrolyte(m(p)), NaCl(m(s))\ AgCl, Ag, where a salt NaCl with different concentrations is added in aqueous poly(diallyl dimethyl ammonium chloride), poly(anethole sulfonic acid, sodium salt) and sodium polyacrylate solutions, respectively. ISE means ion-selective electrode. Mean activity coefficients of the salt NaCl in these aqueous polyelectrolyte solutions were calculated correspondingly. The standard cell potential needed for calculations were obtained from EMF measurements of an another cell, Na-ISE \ NaCl(m)\ AgCl, Ag, where the solution contains only a single electrolyte. Activity coefficients of the electrolyte NaCl in this cell were estimated by Pitzer model. For poly(diallyl dimethyl ammonium chloride) solutions with different concentrations, mean activity coefficients of the salt NaCl decrease monotonically as the concentration of NaCl increases. However, for poly(anethole sulfonic acid, sodium salt) solutions and sodium polyacrylate solutions, the salt-concentration dependence of the mean activity coefficients of NaCl exhibit a maximum.