The Pitzer model can be applied to predict the mean activity coefficients of salts in aqueous electrolyte solutions as a function of the molality. In this work it is shown that the mean activity coefficients for NaOH and NaCl solutions as predicted by the Pitzer model are in good agreement with experimental data from literature. Hereafter, the Pitzer model was extended to the description of the activity coefficients in ion selective membranes. Experimental sorption data of NaOH and NaCl obtained for sulfonic DuPont membranes with EWdry = 1.10 and 1.50, respectively, were used to determine the Pitzer parameters describing the interaction between the fixed charged groups of the membrane and the other ions dissolved in the membrane. In the application of the Pitzer model to the membrane, it was assumed that the fixed charged groups of the membrane can be considered as ＇ordinary＇ ions evenly distributed in the aqueous solution inside the membrane. The results indicate that the Pitzer model can be used to describe the experimental mean activity coefficients in sulfonic DuPont membranes, provided the molality of the membrane, as used in the Pitzer relations, is multiplied by a constant representing a shielding term. This shielding term, f(shielding), is a scaling factor depending on the equivalent weight.