The concept of the elementary cell with isotope reactions has been used for the deduction of the equations giving the relationship between the equilibrium isotopic composition in both solution and resin phases as a function of a few parameters, experimentally controlable. In contrast with the previous works, in this deduction the tendency of the system to the isotopic equipartition with isotope effect has been considered in order to use the formula peculiar to the isotope dilution method. The equations have been expressed in a form suitable for computer calculation of the isotopic abundance variation during chemical and ion exchange isotope separation.