Abstract: In the earlier theoretical treatment, it is usually assumed that all beads of ion-exchanger are spherical and uniform in size.So far the rigorous treatment of isotopic exchange reaction with nonuniform spherical particles has not been given. In this paper, the kinetics of isotopic exchange reaction in systems simultaneously involving ion-exchanger particles of different radii is investigated.The theoretical expressions of polydisperse systems for particle-diffusion control or film-diffusion control have been deduced to predict the fraction of heterogeneous isotopic exchange between ion-exchangers and ions in a solution of finite volume. These expressions have been calculated by computer. It is shown that the smaller particles, reacting more rapidly all the time, will overshoot their eventual equilibrium value and, therefore, they have to release, after some time, a considerable amount of these isotopes already absorbed in favor of the slower reacting particles. Some other conclusions have also been drawn.