Glycidyl methacrylate (GMA) was graft-polymerized onto a porous sheet ( 75% porosity and 1.6-μm average pore size) previously irradiated with an electron beam. The resultant grafted poly-GMA chain can be classified as a polymer brush extending from the pore surface toward the pore interior and a polymer root invading the polymer matrix. The boundary crossing the pore/matrix interface can be detected in two independent ways: molar conversion of the epoxy group into a sulfonic group providing a plateau in the molar conversion versus reaction time curve and molar conversion exhibiting a breakthrough point in the swelling ratio versus molar conversion curve. A higher irradiation dose was found to lead to a higher mole percentage of polymer brush in the graft chain. The dose range from 20 to 200 kGy corresponded to the mole percentage range of polymer brush from 6% to 15%.