The ion-exchange equilibria for the exchange of heavy metal ions (Cu2+, Ni2+, Cd2+ Zn2+, and Co2+) and of Ca2+, Na+, and NH4+ ions by two commercially available chelating ion-exchange resins with aminophosphonate functional groups (Purolite S 940 and S 950) have been studied. From simple binary systems of the exchange of each of the species for H+ ions, the series of selectivity for these species were determined. Evaluation of the binary data by means of the surface complexation theory led to sets of binary equilibrium parameters that remain unchanged in multicomponent systems. By means of these parameter values, ternary and quaternary equilibria and equilibria with complexing-agent-bearing wastewaters from metal surface treatments were predicted and compared with experimental results. In all cases, very satisfactory agreement was found.