Superconducting YBa2Cu3O7-δ oxide specimens (Tc∼91 K) formed by sintering were annealed in He gas ambient at constant temperatures ranging from 300 to 440 °C. Outdiffusion of oxygen during the annealing procedure was monitored by in situ electrical resistivity measurement. Below 350 °C, no resistivity changes were observed with time. From 370 to 440 °C, resistivity increased linearly with annealing time except the very initial period. Indiffusion of oxygen monitored by switching He to O2 (or air) during the annealing occurred extremely rapidly as indicated by a precipitous drop of resistivity. Assuming that the outdiffusion of oxygen is interfacial‐reaction limited, an activation energy of 1.7 eV was measured. The effects of oxygen diffusion on resistivity indicate that oxygen atoms mediate majority carriers in the oxide.