Expanded poly(tetrafluoroethylene) (ePTFE) membranes were modified by graft copolymerization with methoxyacrylethyl phosphate (MOEP) in methyl ethyl ketone (MEK) solutions at ambient temperature using gamma irradiation. The effect of monomer concentration (3-30%) was studied and the modified membranes were characterized by weight increase, x-ray photoelectron spectroscopy (XI'S), scanning electron microscopy (SEM), contact angle measurements, and differential scanning calorimetry (DSC). Results show that the ePTFE membrane had a degree of crystallinity of 59% and that this did not significantly change after grafting indicating that grafting occurs in the amorphous regions. SEM images showed a globular surface morphology for the grafted membranes. XPS was used to evaluate the chemical structure of the graft copolymer and to determine the XPS grafting extent using the C-F (ePTFE membrane) and the C-C (MOEP graft copolymer) peaks. The graft yield as well as grafting extent was found to increase with increasing monomer concentration. Concomitantly, the contact angle was found to decrease with increasing monomer concentration. No direct correlation was found between XI'S grafting extent and the advancing water contact angle illustrating that the former does not adequately give an indication of the copolymer surface coverage of the first molecular layer. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 3331-3339, 2010