Translational energy ( E T ) dependence of oxygen ( O 2) adsorption on clean and modified Cu {100} has been studied by molecular beam surface scattering (MBSS) experiments. The role of surface defects on the adsorption dynamics has been investigated by blocking the defects with Ag and oxygen atoms, and by increasing the defect density with Ar +-ion bombardment. At low E T the adsorption of O 2 is shown to be indirect and facilitated either by a dynamical steering mechanism or a precursor state. At high E T the defects have only marginal effect on the adsorption. Additionally, the modification of the surface electronic structure by preadsorbed oxygen is shown to have a pronounced effect on the adsorption dynamics, whereas the electronic structure modification by preadsorbed Ag is shown to be relatively inefficient in changing the reactivity of Cu {100} towards O 2 adsorption.