Regenerable "gel-coat" chelating resins with fast sorption kinetics and sorption selectivity have application potential for removal of trace metal ions even in large-scale operations. Poly(ethylenimine) has been gel-coated on high surface area silica and insolubilized by cross-linking with a low molecular weight diepoxide (epoxy equivalent 180 g). The sorption of Cr(III)/SO42- on this gel-coated sorbent under vigorous agitation has characteristics of particle diffusion control (pdc) with homogeneous (gel) diffusion in the resin phase. A new mathematical model is proposed for such sorption in gel-coat resin and solved by applying operator-theoretic methods. The analytical solution so obtained shows good agreement with experimental sorption kinetics for relatively low levels of resin conversion.