The objectives of this work were to investigate and compare the separation behavior of ions in single and binary mixed electrolyte systems by NF70, with emphasis on the effect of feed concentration. Experimental results showed that the anion rejection is lower in the binary mixed system compared to the single salt system. Anion rejection decreases with the enhancement of feed concentration of co-existing electrolyte. As for cation rejection, Na+ rejection in the mixed electrolyte system is generally greater than that in the single NaCl system; whereas the separation performance of Ca2+ at high feed concentrations shows the opposite trend, and cannot be explained by the Donnan theory. Several possible mechanisms have been evaluated. Finally, the Spiegler-Kedem equation was used and parameters were calculated.