Experimental studies on the cation-exchange separation of three pairs of divalent metal ions (Zn/Cu, Co/Cu, and Ni/Cu) from water in the presence of water-soluble ligands were made at 298 K. Four ligands including ethylenediaminetetraacetic acid, nitrilotriacetic acid, iminodiacetic acid, and citrate were selected. Experiments were performed as a function of the solution pH (1.0 - 6.0) and the concentration ratio of the ligand to total metals (0 - 1). It was shown that the exchange selectivity was enhanced when the anionic ligands were present. The extent of enhancement strongly depended on the solution pH but was not completely related to the metal-complexing affinity of the ligands. A parameter, based on the combinations of the overall formation constants of the ligands with two metals, was thus proposed to correlate the exchange selectivity obtained under various sets of metal pairs and anionic ligands. This correlation allowed selection of a suitable type or amount of the anionic ligands for effective separation of a given pair of heavy metals.