AbstractA study has been made of the cation exchange equilibria between Dowex 50W‐X8 resin and aqueous solutions of copper nitrate, nitric acid, and sodium nitrate in binary and ternary mixtures. Experiments were carried out at room temperature with solutions having total cation concentrations of approximately 0.1N, 0.05N, and 0.01N. It was found that the equilibria for any two ions were essentially the same in binary and ternary mixtures. Moreover single valued selectivity coefficients were determined which characterize these data quite well for all three concentration levels. This makes possible the complete description, for engineering purposes, of ternary equilibria in terms of the simpler and more accessible binary exchange reactions, as previously suggested (1