To clarify the thermochemical behavior of oxygen in SnO2-doped In2O3 (ITO) powders an evolved-oxygen gas analysis (EGA) and successive oxidations were carried out by heating and cooling of the samples under a constant oxygen partial pressure. The rates of oxygen released and gained were analyzed as a function of temperature and time. The results implied that the ITO phase is a mixed state of various microstructures, phi(T), although it essentially possesses a C-type rare earth structure. Diffusion of oxygen in ITO occurred smoothly : while the attainment of a single phi(T) structure required many hours; the oxygen concentration in ITO could not be fixed unequivocally by a set temperature and oxygen partial pressure. The effect of the mixed state of phi(T) on the kinetic behavior of oxygen was also examined.