The paper presents experimental results on an in situ optical absorption study of reduction/oxidation processes in YAlO 3 . The increase in the absorption of the bands at 22,000 and 40,000cm –1 during annealing in an oxidizing atmosphere as well as their bleaching during reduction were studied in YAlO 3 and YAlO 3 :Nd crystals in the temperature range 600–1000K. The oxidation and reduction kinetics were analyzed by means of exponential decay modeling as well as in the framework of different models, taking into account diffusion and quasi-chemical reactions of the phototropic centers.