In situ resistivity measurement has been used to monitor diffusion of oxygen in and out of bulk and polycrystalline YBa2Cu3O7- in ambient O2 and He atmospheres. The out-diffusion of oxygen from the oxide is found to be independent of and its rate is surface-reaction limited with an activation energy of 1.7 eV. The in-diffusion depends strongly on; the activation energies of the process at 0 and =0.38 are 1.1 and 0.48 eV, respectively. The latter is taken to be the activation energy of motion and the former, the activation energy of diffusion. On the basis of the kinetic and published structural information, we have proposed a defect mechanism in the CuO plane for the anisotropic diffusion of oxygen in the oxide.