The ion-exchange isotherm of sodium and calcium ions and membrane potential measurements of monovalent salts (KCl and NaCl) and monovalent-divalent cation salt mixtures on the polysulfone and polysulfone with polyester support cation exchange membranes have been performed. The equilibrium cation concentration in membranes for monovalent-divalent (NaCl + CaCl2) cation salt mixtures were studied at a total concentration of 0.05 M. The electromotive force values of the cell-Ag,AgCl\MCl(a')\ membrane\MCl(a")\Ag,AgCl for monovalent salts and Ag,AgCl \MCl+CaCl2(a')\membrane\MCl +CaCl2(a")\Ag,AgCl for monovalent-divalent salt mixtures-were measured. A nonlinear regression method was used to determine effective membrane charge densities. The membrane potential of the monovalent salts for cation exchange membranes is almost linear. Membrane potential values for monovalent-divalent cation mixtures are not linear as a function of NaCl concentration and tend to be zero when the mole fraction of monovalent-divalent salts is equal. The results obtained for polysulfone-type membranes were compared with Neosepta CL-25T and CM2 cation exchange membranes.