The effect of ionic strength variation on the ion exchange equilibrium between AMX anion exchange membrane and electrolyte solutions containing the most dominant anions on natural waters (Cl−, NO3−, and SO42−) was studied. All experiments were carried out at a constant temperature of 25 °C. Ion exchange isotherms were established, at different ionic strengths from 0.1 to 0.5 M, for the systems (Cl−/NO3−), (Cl−/SO42−) and (NO3−/SO42−). Obtained results showed that for I = 0.1 M the affinity order is SO42 −>NO3−>Cl−. For I = 0.2 M this order is NO3−>SO42−>Cl−. For 0.3 and 0.5 M the AMX membrane becomes more selective for chloride than for nitrate or sulfate. Selectivity coefficients KNO3−Cl−, K2Cl−SO42− and K2NO3−SO42−, thermodynamic constants, and separation factors were calculated and decreased with the increase of ionic strength.