It was shown that flat elution curves were maintained with constant length rare-earth bands using either copper-ammonium- hydrogen or copper-hydrogen retaining beds. However, when the copper-hydrogen retaining band was used, some precipitation occurred at the lower boundary region or the rare-earth band. When the copper-ammonium-hydrogen band was used, a longer time was required to achieve equilibrium in this bend. When a narrow copper-ammonium-hydrogen bend was used, between the rare-earth and the copper-hydrogen bed, the precipitation was prevented and the "tlat" equilibrium type elution curves were obtained. It was demonstrated that when a final retaining bed of the proper composition is used with a given eluant, then the interposed band and the rare-earth band maintain a. constant length. In addition, it was demonstrated that, when the final retaining bed was not compatible with the eluant, then the flat type elution curves were maintained with the interposed band growing in length and the rare-earth band remaining constant in length.