Adsorption dynamics experiments on Cu( 100) at 300 K indicate that surface defects induced by 3.4 keV Ar+-ion bombardment strongly enhance the dissociative chemisorption probability of O-2. Energy selective molecular beam surface scattering experiments reveal a defect induced low-barrier dissociation pathway leading to enhanced dissociation of O-2 molecules with translational energy up to 60 meV. Density functional theory calculations attribute the decrease in O-2 dissociation barrier to the weakening of O-O bond at Cu vacancies, thus resolving the contradiction between experimental observations and theoretical predictions of the height of the barrier to O-2 dissociation on Cu( 100). (c) 2008 Elsevier B. V. All rights reserved.