The surface composition of radiation-grafted FEP-g-poly(styrene-co-divinylbenzene) films and its sulfonated analogues was investigated by X-ray photoelectron spectroscopy. These investigations demonstrate that the surface composition is strongly influenced by the degree of crosslinking of the polystyrene phase. In the absence of a crosslinker, the surface concentration of the grafting component was high and could even exceed the calculated average concentration in the volume of the polymer. With increasing cross-linker concentration, the surface concentration of the grafting component decreased rapidly and became close to zero for crosslinker concentrations above 2% in the monomer mixture. The effect was most pronounced in the case of grafted films and weaker in the case of sulfonated membranes. (c) 2007 Wiley Periodicals, Inc.