AbstractWe estimated surface penetration barriers for interstitial oxygen using a simple calculational scheme, the effective-medium theory for a single atom. In this theory the single parameter describing the host is the electron density. Since the electron density depends primarily on the interatomic distances, i.e. the structure of the host, we found that the barrier to oxygen penetration in oxides depends primarily on the structure of the oxide, the closest-packed isotropic oxides with the smallest lattice constants having the highest barriers.