In order to improve the selectivity of the AMX membrane, a polyethyleneimine (PEI) layer was adsorbed on its surface by the immersion of the considered membrane in a solution of PEI. The adsorption process of PEI was studied and the results were well correlated with the Langmuir model and with Lagergren's pseudo-first-order kinetic model. Equilibrium ion exchange isotherms of Cl-/NO3-), (Cl-/SO42-), and (NO3-/SO42-) systems, using modifiedAMX membrane, were established at various ionic strengths from 0.1 to 0.5 mol/L and at constant temperature of 298 K. The results obtained with the modified membrane, in this range of ionic strengths, showed that the affinity order was: NO3- > Cl- > SO42- at I = 0.1 mol/L and I = 0.2 mol/L and it was Cl- > NO3- > SO42- for I = 0.3 mol/L and I = 0.5 mol/L. The ion exchange selectivity coefficients K-A(B), thermodynamic ion exchange equilibrium constants K-A degrees(B), and the standard free enthalpies Delta(r)G degrees of the ion exchange reactions were determined for the three binary systems. These results were compared with those obtained with the unmodified membrane. It was observed that for the modified membrane the ion exchange selectivity towards sulphate ions decreased and increased towards monovalent anions.