Successful implantation of any biomaterial depends on its mechanical, architectural, and surface properties. Materials with good bulk properties seldom possess the appropriate surface characteristics required for good biointegration. The present study investigates the results of surface modification of a highly porous, fully fluorinated polymeric substrate, expanded poly(tetrafluoroethylene) (ePTFE), with a view to improving the surface bioactivity and hence ultimately its biointegration. Modification involved gamma irradiation-induced graft copolymerization with the monomers monoacryloxyethyl phosphate (MAEP) and methacryloxyethyl phosphate (MOEP) in various solvent systems (water, methanol, methyl ethyl ketone, and mixtures thereof). In order to determine the penetration depth of the graft copolymer into the pores and/or the bulk of the ePTFE membranes, angle-dependent X-ray photoelectron spectroscopy (XPS) and magnetic resonance imaging (MRI) were used. It was found that the penetration depth was critically affected by the choice of monomer and solvent as well as by the technique used to remove dissolved oxygen from the grafting mixture: nitrogen degassing versus vacuum. Difficulties due to the porous nature of the membranes in establishing the lateral position of the graft copolymers were largely overcome by combining data from microattenuated total reflectance Fourier transfer infrared (μ-ATR-FTIR) mapping and time-of-flight secondary ion mass spectrometry (ToF-SIMS) imaging. Results show that the large variation in graft heterogeneity found between different samples is largely an effect of the underlying substrate and choice of monomer. The results from this study provide the necessary knowledge and experimental data to control both the graft copolymer lateral position and depth of penetration in these porous ePTFE membranes.