We investigate how the initial formation of an oxide layer on a metal surface can effect the further incorporation of oxygen into the metal. In particular, we show that the formation of NbO on a Nb surface dramatically increases the potential barrier for oxygen incorporation. On Cu the formation of an oxide layer reduces the barrier. On Cr and on Ni the barriers on the metal and on the oxide are similar. It is shown that the height of the barrier can be significantly reduced by the relaxation of the atoms in the surface oxide. The gain in energy in this process is shown to depend on the magnitude of the elastic constants of the oxide on the surface. We also show that oxides which grow by Cabrera-Mott Kinetics have a barrier to oxygen penetration while others do not. © 1987 The American Physical Society.