The resistivity variation in ceramic YBa2Cu3O7-α (0≤δ<0. 8) oxides has been studied as a function of δduring the oxygen in-diffusion and out-diffusion processes. The distribution of δ=δ(x,t) as a function of the position (x) and time (t) within a grain has been inferred for both processes. We conclude that the oxygen distribution in the oxides is inhomogeneous in the beginning of in-diffusion. This inhomogeneity is due to the fast saturation of oxygen (i.e., δ=0) in the outer layer of each grain and the high activation energy of diffusion of oxygen through these superconducting "shells" (E=1.3±0.1 eV), which limit the rate of forming a uniform sample. The shell formation has important effects on the electronic properties of these superconducting oxides, and must be taken into account in attempting to control the oxygen homogeneity in these oxides, especially in single crystals. On the other hand, out-diffusion produces much more homogeneous samples of δ>0 due to a surface energy barrier Eout=1.7±0.1 eV. © 1988 The American Physical Society.