We have used in situ resistivity measurements to monitor the diffusion of oxygen in and out of the ceramic oxide YBa2Cu3O7-. The study of out-diffusion of oxygen was carried out by annealing the oxide in ambient helium at constant heating rates and at constant temperatures. The rate of out-diffusion of oxygen has been found to be independent of , and the resistivity changes linearly with time during isothermal anneals. Assuming that the rate of out-diffusion is surface reaction limited, we have determined a surface barrier of 1.7 eV. The study of in-diffusion of oxygen was performed by annealing the oxygen-deficient oxide (=0.38) in ambient oxygen. The in-diffusion rate was found to depend strongly on ; we have determined the activation energies of the process at =0.38 and =0 to be 0.5 eV and 1.3 eV, respectively. The diffusivity of oxygen for =0 has been determined to be D=0.035 exp(-1.3eV/kT) cm2/sec. Combining the kinetic data and published structural information, we have examined a vacancy mechanism and a twinning mechanism for anisotropic diffusion of oxygen in the CuO layer between the BaO layers. © 1989 The American Physical Society.