Vacuum-annealed samples of single-phase orthorhombic YBa2Cu3O6.9 have been reoxygenated. The vacuum-annealed material is tetragonal, semiconducting at room temperature, and shows no superconductivity. Upon annealing these samples in 640 Torr O2 at 200°C, a partial return of 90 K superconductivity is observed; complete restoration is achieved following 300°C anneals. Oxygen uptake in vacuum-annealed pellets follows an Arrhenius relation with an activation energy of −27 kcal/mol. Deoxygenation and reoxygenation does not seem to affect the structure of the grain interconnects in the material. The maximum oxygen uptake for vacuum-annealed material occurs at 450°C, while the maximum magnetic Meissner effect is observed following oxygen annealing at 350°C.