Experimental results indicate effective faster diffusion of oxygen in polycrystalline alumina when exposed to water vapor at high temperatures. α-Al2O3 containing Ni metal particles was exposed to dry air or humid environments at 1300°C for up to 20 h. Oxidation of Ni in α-Al2O3 to form NiAl2O4 was used to determine oxygen diffusion depth from the surface. The apparent kinetic rate constant for oxygen diffusion in the presence of water vapor was 79% higher compared with that in a dry atmosphere at 1300°C (1.4 × 10−13 m2/s for 0.2 atm versus 7.9 × 10−14 m2/s for dry air). (OH)− ions are smaller and more polarizable than O2−, which would be expected to lead to faster diffusion. This effect may impact sintering, creep, corrosion, oxidation, and the performance of thermal barrier coatings (TBC).