In this work, the effects of Ar+ ion beam irradiation on carbon fibers were studied using tensile and surface analytical techniques. The single-fiber pull-out test was executed in order to characterize the fiber/epoxy matrix interfacial adhesion. The Ar+ ion beam was irradiated using an ion-assisted reaction (IAR) method in reactive gas conditions under an oxygen environment with 1 X 10(16) ions/cm(2) Ar+ ion dose (ID), 6 seem blown gas flow rate, and different ion beam energy intensities. From the experimental results, both the interfacial shear strength (IFSS) and fracture toughness (G(i)) were found to increase with increasing Ar+ ion irradiation intensity. This was probably due to the fact that Ar+ ion beam irradiation on carbon fibers was effective in altering their surface physical chemistry and structural morphology, resulting in improved interfacial adhesion in the fiber/epoxy matrix. The reliability of single-fiber pull-out test data could be improved by statistical analysis using the Weibull distribution, which served to predict the variation of the mechanical interfacial properties in a composite system. (C) 2003 Elsevier Science (USA). All rights reserved.