The diffusional process in the chlorination of sulfonated poly(styrene-co-divinylbenzene) bead was investigated through a general model as well as direct observation with electron microprobe. Two modes of diffusion, namely the pore diffusion and gelular permeation, were analyzed by two system parameters through a two-phase model. The effective pore and gelular diffusivities estimated from the x-ray energy-dispersive analysis data on the chlorine profile and the conversion suggest that, even for the same crosslinking, the diffusivity of chlorine in the gelular bead is larger by several orders of magnitude than that for the gelular microparticle in the macroreticular bead.