The intraparticle diffusions of H3PO4, H2PO4-, HPO42-, and PO43- ions in an OH-type strongly basic ion exchanger, DIAION SA10A, were investigated by measuring the uptake curves for adsorption of phosphates by using the shallow-bed method. The experimental uptake data were correlated by applying the homogeneous Fickian diffusion model and the parallel diffusion model for surface and pore diffusions. In all phosphate systems, the values of intraparticle effective diffusivity (D-eff) obtained from the homogeneous model increased with the increase in the bulk-phase concentration, and suggested the existence of a parallel transport of phosphates in the particle by the surface and pore diffusions. The surface diffusivities (D-s) for the parallel diffusion model were determined from the intercepts of the plots of D-eff[l + (epsilon(p)C(0)/q(0))] vs. epsilonpC(0)/q(0). The accurate pore diffusivities (D-p) were obtained by matching the theoretical uptake curves for the parallel diffusion model with the experimental uptake data. The parallel diffusion model agrees reasonably well with the experimental uptake data. (C) 2004 American Institute of Chemical Engineers.