The concept of a theoretical plate has been applied as a column operation parameter to evaluate the effects of flow rate and temperature on the separation of erbium and thulium with HEDTA using an ion-exchange technique. It has been shown that for separations made at constant temperatures the overlap between the pure forms is directly proportional to the flow rate at all practical flow rates and temperatures. The changes in the HETP were easily explained in terms of the ion-exchange kinetic theory. We would expect a decrease in the HETP for any other change in the system which would facilitate the diffusion of ions through the resin particle, i.e., resin of small particle size or a resin of low cross-linkage.