Transparent lanthana‐doped yttria fabricated by transient solid second‐phase sintering under wet hydrogen typically has a broad absorption band with a peak at 3.08 μm. The absorption band shift observed in samples treated in wet deuterium indicated that the 3.08‐μm absorption was due to OH−ions. The diffusion rates of hydrogen defects in lanthana‐doped yttria were determined in the temperature range from 1000° to 1400°C. The changes in the concentrations of OH−ions upon anneals were determined by measuring infrared absorbance at 3.08 μm. The diffusion coefficient is 1.3 × 10−7, 9.9 × 10−7, and 4.1 × 10−6cm2/s at 1000°, 1200°, and 1400°C, respectively, with an activation energy of 140 kJ/mol. Annealing in a controlled oxygen partial‐pressure environment can remove the OH−absorption band and bring the total absorption in the 3‐ to 5‐μm