The emf values of the cell Ag,AgClKCl(a’) membraneKC(a")Ag,AgCl have been measured. Two types of cellulosic strong ion-exchange membranes were tested, containing either cationic or anionic fixed groups. The membranes were prepared by grafting acrylic monomers onto paper followed by opening of epoxy groups. The membrane phase was analysed for its electrolyte and water contents, and intramembrane activity coefficients were derived as a function of external electrolyte concentration. The data were used for calculation of the emf of membrane cells using the TMS theory reformulated by Hills et al. The main finding of this investigation is that, for dilute external solutions, the membrane potentials are close to the maximum theoretical value and can be calculated with reasonable accuracy using the fixed charge theory. With a more concentrated solution this is not so; the membrane potentials are progressively smaller than the maximum value and the theory did not give satisfactory agreement between the observed and the calculated emf of the membrane cells. This is readily explained by the decrease in the selectivity of the membranes, the increasing concentrations of co-ions, and the diffusion of electrolyte through the membrane.