Mixed solutions containing copper salts were separated using a nanofiltration membrane. For mixed electrolyte solutions with the same co-ion, the rejection of copper was shown to depend strongly on the nature of the second counter-ion. In the presence of H3O+ or at low concentration of Na+, copper ions are completely rejected, while in the presence of Ca2+, their transfer is slightly improved. The extended Nernst Planck equation was applied to modelize experimental results using specific calculated transfer parameters. The variations of the concentration and the electrostatic potential were considered to be linear through the membrane. Moreover, the solute concentration near the membrane was assumed to be equal to the feed solution concentration. Calculated copper rejections are in good agreement with experimental results.