The work presents experimental results of an in situ investigation of optical absorption of LiNbO(3) : MgO during reducing (95%Ar + 5%H(2)) and oxidizing (O(2)) high-temperature treatments in the temperature range from room temperature to 1073 K. The absorption spectra measured at in situ conditions at high temperatures in reducing/oxidizing atmospheres as well as the kinetics recorded at fixed wavelengths during rapid replacement of gas atmospheres have been analyzed. The origin of the changes in optical absorption caused by the reducing/oxidizing treatments is discussed in terms of hydrogen and oxygen ion diffusion and the point defect structure of the material.