The ion separation fctors for 21 common metal ions with cross-linked polyacrylic acid were determined as a function of pH and the percent of the cross-linked polyacrylic acid neutralized. The calcium ion was used as a reference. At a pH of 5, the decreasing order of affinity of the ions for the cross-linked polycrylic acid was found to be: Hg** plus ** plus , Fe** plus ** plus ** plus , Pb** plus ** plus , Cr** plus ** plus ** plus , Cu** plus ** plus , Cd** plus ** plus , Al** plus ** plus ** plus , Ag** plus , Zn** plus ** plus , Ni** plus ** plus , Mn** plus ** plus , Co** plus ** plus , Ca** plus ** plus , Sr** plus ** plus , Ba** plus ** plus , Mg** plus ** plus , K** plus , Rb** plus , Cs** plus , Na** plus , and Li** plus . Members of a chemical family exhibited similar selectivities. The Hg** plus ** plus ion appeared to be about a million times more strongly bound than the alkali metal ions. The relative binding of most of the metal ions varied with pH; the very tightly and very weakly bound ions showed the largest variations with pH. The calcium ion - hydrogen ion equilibrium was perturbed very little by the presence of the other ions. The separation factors and selectivity coefficients are discussed in terms of equilibrium and thermodynamic significance.