Abstract: The dependence of rates of the heterogeneous isotopic exchange reactions of Eu (Ⅲ) between the cation-exchange resin and aqueous solutions on the concentrations of hydrogen ion, citric acid and acetic acid has been investigated. It is shown that hydrogen ion has an obvious catalytic effect.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 parameter, ξ. The self-diffusion coefficients D of Eu (Ⅲ) in these DTP A aqueous solutions were also determined by open-end capillary method. From ξ, D and D, the thickness of liquid film δ was estimated.In addition, the same aqueous solutions were investigated with spectrophotometric and pH titrimetric method but no new mixed-complex was fonnd.On the basis of our experimental results, we studied the actions of H+, HaCit, HAc and discussed the works of Chmutov and that of Spitsyn and Martynenko.We propose that δ may be regarded as a criterion, which shows roughly the effects of the other chemical reaction on the over-all ion exchange rate.