The synergistic effect of atomic oxygen and vacuum ultraviolet (VUV) radiation exposures was investigated on the mechanical properties of high-modulus aramid fibers. Tensile tests were performed on single fibers. It was found that both tensile strength and Young's modulus decreased significantly due to the simultaneous exposures of atomic oxygen and vacuum ultraviolet radiation. In contrast, the surface of aramid fiber exposed to ultraviolet alone showed no change in tensile strength and Young's modulus. X-ray photoelectron spectroscopy showed that atomic oxygen exposure leads to oxidization and decomposition of amid groups, that only VUV exposure does not change the fiber surface, and that VUV exposure decreased oxidized components on the oxidized fiber surface exposed to atomic oxygen and ultraviolet simultaneously. Scanning electron microscopy revealed that the diameter of the fiber was not influenced so much by atomic oxygen and ultraviolet exposures, but the fiber surface became substantially rougher than the aramid fiber surface exposed to atomic oxygen alone. (c) 2006 Elsevier Ltd. All rights reserved.