Simple mathematical models for binary and ternary ion exchange equilibria were developed to predict selectivity-coefficients and resin capacities from a simple experiment to within acceptable engineering accuracy limits. Batch equilibrium experimental investigations were undertaken to Lest the models for binary systems (Ni-Na, Cu-Na, Zn-Na, Cd-Na and Ph-Na) and ternary systems (Ni-Cu-Na, Cd-Ni-Na, Cd-Cu-Na and Pb-Cu-Na) with a commercial DOWEX HCR-W2 resin. Binary parameters were used to predict ternary equilibria. It was found that good agreement between the predicted and experimental values was obtained when the binary selectivity coefficients of the two competing heavy metal ions in the ternary system were either almost equal or differed by a factor as large as five.