The chromatographic plate theory is applied to the displacement development on ion exchangers of a band containing a mixture of two very closely related species. When the number of plates of the band P is small and the separation factor α=1 + ϵ is closie to unity so that P, ≪ 1, and if one of the species of the mixture is of a small percentage, then a linear distribution of the concentrations in the band is obtained. In a more general case the distribution's eqution is given. The band is symmetrical only if its initial composition is 50–50. In every other case the plate corresponding to the initial mixture is not the median plate of the band. We have derived a mathematical expression which gives the place of the median in the band. A practical determination of the distribution of species in a steady-state band, when P, ϵ, and the composition of the mixture are known, is given. A limited experimental verification is given for the case of the separation of boron isotopes.