Cation-exchange separation of Co(2+)/Ni(2+) from aqueous solutions using water-soluble complexing agents of ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), iminodiacetic acid (IDA), and citrate was experimentally studied at 298 K. Experiments were carried out as a function of initial aqueous pH (1.0-6.0), concentration of total metals (1.5-45.0 mol/m(3)), the concentration ratio of two metals (0.1-10) and of complexing agent to the total metals (0-1). It was shown that the exchange selectivity strongly depended on solution pH and was not completely related to the affinity of any metal with the complexing agents. When a certain level of complexing agent was present, highly effective separation could be achieved at an appropriate pH range (for an equimolar metal solution, e.g., pH 2-3 with EDTA and NTA as well as pH>3 with IDA and citrate). The application potential of this method was highlighted for the separation of Co(2+) from binary mixtures in the presence of trace amount of Ni(2+) due to its high selectivity and the smaller amount of the complexing agents needed.