The oxides of oxygen‐deficient YBa2Cu3O7−δ with δ≂0.32 in both powder and chunk form were annealed in ambient oxygen to investigate the behavior of oxygen indiffusion. Thermogravimetric measurements were used to monitor in situ weight gain during the annealing at constant heating rates between room temperature and 580 °C. The temperature for achieving a definite oxygen stoichiometry changes with the heating rates, which were varied from 0.1 to 15 °C/min. By measuring the rate dependence, the activation energy of oxygen diffusion in the powder oxide was determined to be 1.25±0.05 eV, in good agreement with the value obtained by previous resistivity measurements. However, the activation energy obtained from the chunk oxide is higher, i.e., 1.52±0.06 eV. A longer diffusion distance in the chunk oxide which requires diffusion across grains might be responsible for this discrepancy.