Abstract: The rate and mechanisms of the heterogenous isotopic exchange reactions of Eu (Ⅲ) between cation-exchange resin and aqueous solution in presence of chelating agent (DTPA or NTA) have been investigated. A superior, new theoretical equation for a composite sphere in a limited bath was employed in computer-assisted, nonlinear least-square fits of the experimental rate data, in order to obtain accurate particle self-diffusion coefficients, D, and values- of the kinetics mechanism parameter, |. Estimates of the thickness of liquid film, δ, were obtained.It is found that, in the presence of NTA, these experimental results agree quite well with those predicated by the theoretical equation; but a certain discrepancy between experimental and theoretical values exists in the presence of DTPA.It is pointed out that the discrepancy could be attributed to the slow rate of the dissociation of the Eu (Ⅲ)-DTPA complex.The results indicated that NTA is better than DTPA for the separation of rare earths by displacement chromatography not only for more rapid rate of diffusion of species in the resin, but also for more rapid rate of the dissociation of complex in the solution.Assisted by computer, a new table of Bt corresponding to the values of F has been prepared, which is more precise than the one calculated by Reichenberg.