Zn2+, Cu2+, and Ni2+ ionic permeabilities through a Nafion 115 membrane, containing sulfonate functional groups capable of cation exchange with metal ions, were measured in this work. Single and binary salt mixtures were prepared at 10, 100, and 1000 ppm concentrations, and the ionic permeabilities and selectivities (alpha) were measured. The ionic permeabilities were in the 10(-6)-10(-5) cm(2)/s range, and the order of ion permeability depends on the ionic concentration. At low (10 ppm) concentration, the smaller Ni2+ transported faster than the other two ions and the permeability was dominated by the ionic diffusivity. As the concentration was increased to 100 and 1000 ppm, the ion permeability was increased because more ions were exchanged onto the sulfonate groups. Moreover, Zn2+ became the fastest ion among the three as the ion transport became solubility-controlled at the medium and high concentrations. In spite of the ion permeability order being dependent upon the feed concentration, the separation selectivity was not affected by the presence of co-ions in the tested conditions. The separation selectivity from a binary mixture could be estimated from the permeability ratio obtained from single solutions.