Abstract: In this paper, the kinetics of isotopic exchange reaction in systems simultaneously in-volving ion-exchanger particles with different radius is investigated. A general expression for polydisperse systems under both particle-diffusion and film-diffusion control has been theoretically deduced to predict the fraction of heterogeneous isotopic exchange between ion-exchangers and ion in a solution of finite volume. From the expression mentioned above previous theoretical expression on heterogeneous isotopic exchange may be derived.The expression deduced in this paper supplements what is lacking in the mathematical model proposed by Huang and Li, which assumes that all beads of ion-exchanger are spherical and uniform in size.With this expression, some numberical calculations have been done with a computer and some conclusions drawn..