The zero-length column (ZLC), conventionally developed for measuring the diffusivity of gases and liquids in zeolite crystals, has been extended to the measurement of intraparticle diffusivities in ion exchangers. A theoretical model which considers the ZLC cell as a continuous stirred tank. reactor is developed. The model allows the use of nonlinear ion-exchange equilibrium isotherms, describes ion fluxes with Nernst-Planck equations, and includes the resistances to mass transfer both in the particle and in the film. Simulations have been carried out to analyze the effect of model parameters on the system response and to choose experimental operating conditions. The model validity has been tested by comparison with the experimental results of the ZLC for a step perturbation in the binary systems Na+-H+, K+-H+, and Na+-K+.