The mechanisms of the photoactivation of the surface of "real" Al₂O₃ in sub-band-gap region, the nature of active surface centres, the electronic and atomic structures of oxygen species on UV-irradiated surface have been studied step-by-step. The variety of complementary experimental methods have been adapted to carry out in situ investigations in three phases: gas-adsorbate-surface. Specific oxygen monoatomic, diatomic and three-atomic oxygen species on UV-irradiated Al₂O₃ surface have been revealed.