Adsorption dynamics experiments on Cu(100) at 300K indicate that surface defects induced by 3.4keV 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 60meV. 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).