Abstract We report on the thermal treatment effects in a γ-ray irradiated oxygen deficient amorphous silicon dioxide (a-SiO 2) containing Al impurities. We observed that by thermal treatments the intensity of the 7.6 eV optical absorption band, associated to an oxygen deficient center, and the EPR signal amplitude of irradiation induced [AlO 4] 0 centers gradually decrease. During these thermal treatments, the E γ ′ centers concentration is found to increase in a correlated way to the decrease of the 7.6 eV absorption amplitude. These results are interpreted assuming an hole-transfer process from the [AlO 4] 0 centers to the diamagnetic oxygen vacancies, resulting in the generation of E γ ′ centers. Our results prove the oxygen vacancy model for the 7.6 eV optical absorption band and its precursor nature for E γ ′ centers.