In order to improve the selectivity of the AMX membrane, a polyethyleneimine (PEI) layer was adsorbed on its surface. Ion exchange isotherms of (Cl-/NO3-), (Cl-/SO42-) and (NO3-/SO42-), using a modified AMX membrane, were established at different temperatures (from 283 to 313K), with a constant total concentration of 0.3mol/L and neutral pH. The results obtained with the modified membrane in this range of temperature show that chloride was the most sorbed and the selectivity order was Cl->NO3->SO42-. Selectivity coefficients and thermodynamic equilibrium constants, determined for the three binary systems, increased with increasing temperature. These results were compared with those obtained with the unmodified membrane. It was observed that for the modified membrane, the selectivity towards sulphate ion decreased and the modified membrane became more selective towards monovalent anions. Ion exchange equilibrium at temperatures 283, 298 and 313K of the studied binary systems were found to be endothermic processes.