We have annealed the oxygen-deficient ceramic oxide YBa2Cu3O7-δ, where δ=0.4, in ambient oxygen to obtain in-diffusion of oxygen. In situ resistivity measurements were used to monitor annealing at constant heating rates between room temperature and 570 °C. The resistivity curve showed a maximum and a minimum. The temperatures where the maximum and the minimum were observed change with heating rate. By measuring their dependence on heating rate, which was varied from 15 to 0.1 °C/min, we have determined the activation energy of oxygen diffusion in the oxide (δ=0) to be 1.35±0.10 eV. The in-diffusion is limited by a near-surface layer which requires the measured activation energy for oxygen to diffuse through it.