The parallel transport of an organic acid by solid-phase and macropore diffusion within a porous ion exchanger was studied by measuring equilibrium isotherms and uptake curves for adsorption bf acetic acid and lactic acid on a weakly basic ion exchanger, DIAION WA30. Experimental adsorption isotherms were correlated by the Langmuir equation. The Langmuir equilibrium constant of acetic acid was close to that of lactic acid, and the saturation capacity of acetic acid was about 84% that of lactic acid. Intraparticle effective diffusivity D-eff was determined using the homogeneous Fickian diffusion model. The value of D-eff for acetic acid was about 1.5 times lactic acid. Because D-eff increased with linearly increasing bulk phase concentration C-0, D-eff was separated to the solid-phase diffusivity D-S and the macropore diffusivity D-P by applying the parallel diffusion model. The model agreed well with the experimental curves. The values of D-S and D-P for acetic acid were about 2 and 1.5 times those of lactic acid, respectively. The acetic acid and the lactic acid may be separated by the difference of the diffusion rates.