Radiation-induced graft polymerization of styrene into crystalline poly(ether ether ketone) (PEEK) with 32% crystallinity, an aromatic hydrocarbon polymer and so-called "super-engineering plastics", which has excellent mechanical properties at elevated temperature as well as excellent gas barrier properties, was investigated with differential scanning calorimetry (DSC), thermogravimetry (TGA), X-rays diffraction analysis (XRD), and electron spin resonance spectroscopy (ESR). The endothermic heats of melting of the original and styrene-grafted PEEK (grafted PEEK) films were similar, indicating that the crystallinity was almost completely maintained during the graft polymerization of styrene up to a grafting degree of 51%. Lower glass transition temperature (T-g) of the grafted PEEK film in the DSC profile, and the absence of an extra halo originating from amorphous polystyrene grafts in the XRD strongly indicate that the grafting of styrene to crystalline PEEK films proceeded in the amorphous region of PEEK. This is probably because polystyrene grafts have hydrocarbon structures similar to a base PEEK polymer, resulting in compatibility to the amorphous phase of the PEEK films. The grafted PEEK films can be converted to PEEK-based electrolyte membranes (PEEK-based PEM) by subsequent sulfonation of the polystyrene grafts. The PEEK-based PEM had conductivity of more than 0.01 S/cm and exhibited higher water content above 100%. (C) 2009 Elsevier B.V. All rights reserved.