Ultra-high molecular weight polyethylene (UHMW-PE) has a highly chain-extended and crystalline structure which is functionally inert and requires surface-modification before it can successfully operate as a reinforcement in polymer-matrix composites. Although plasma treatments are adequate for this purpose, recent work has shown that irradiation with low-energy inert gas ions can produce increases in interfacial shear strength (ISS), in epoxy matrices, which exceed those of commercial plasma treatments, and cause little degradation in tensile properties. Low energy ions are readily produced in high-current beams using gridded sources having moderate cost, and processing times may be a short as a few seconds. In this paper, we present results of recent experiments using argon ions accelerated to energies between 100 eV and 1 keV to irradiate 20–30 μm diameter UHMW-PE fibers to doses between 1×1016 and 1×1017 cm−2, and compare our findings with previous work at higher accelerating potentials. At the optimum dose (which increases with decreasing energy), greater than 9-fold improvements in ISS level, measured in epoxy-resin droplet pulloff tests, were found for ion irradiation at 0.25 keV. Scanning electron microscopy of fiber surfaces, of ion irradiated as well as commercial oxygen plasma-treated materials, revealed small crack-like pits in both cases, with the pits smaller and more uniformly distributed on the ion-irradiated fibers. Surface chemistry studies using X-ray photoelec-tron spectroscopy (XPS) and attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) indicate that irradiation resulted in high surface concentrations of polar functional groups, and extensive surface oxidation. This was accompanied by a substantial increase in the polar component of surface energy, which resulted in improved fiber wetting by the resin.