Membrane potentials of polysulfonated cation exchange membranes (unsupported polysulfone, SA3S and polyester supported polysulfone, SA3T) for KCl, NaCl and LiCl solutions using Ag/AgCl electrodes were measured. The measurements were carried out for, KCl SA3S or SA3T MCL, (M: K, Na and Li) systems. The concentration of salt (KCl) was kept constant in one side of the cell, C1, at 1.10-2 M was changed in the other side of the cell C2, from 1.10-4 to 2.0 M. The alteration graphs were drawn in order to find the potential values of membranes for ( ln C2) changes. Using the Donnan equation and the Teorrel Meyer Sievers theory, Donnan potentials were calculated theoretically. It was found that it had been dependent on ionic concentration of external solution. Donnan potential values determined for both membranes are parallel at 0.05 M concentration but there is deviation this value.