The absorption spectra of unirradiated and irradiated (to fast neutron fluence 10 12 and10 14 cm −2 in VVR-SM) quartz-quartz fibers after additional irradiation with 60 Co γ rays to 10 8 rad are investigated. It is shown that neutrons transform E ′ centers into ≡Si-Si≡ centers. The kinetics of accumulation of bridge oxygen atoms under the action of γ radiation exhibits two stages. The first stage is due to neutron irradiation and the manifestation of preliminary potential centers of nonbridge oxygen atoms. The second stage is associated with the creation of additional nonbridge oxygen atoms under the action of γ radiation. The γ-ray dose for the onset of the second stage decreases as the fluence of the preliminary neutron irradiation increases.It has now been determined that different structural defects, whose concentration varies over wide limits, appear in quartz fiber lightguides depending on the content of impurities and OH groups, type of cladding, and drawing conditions [1-3]. Consequently, fiber lightguides are convenient objects for studying the influence of structural defects on defect formation during subsequent irradiation. The results of the investigations of radiation-stimulated processes in fiber lightguides are necessary to improve the theory of radiation damage and can be used to develop methods to increase the radiation resistance or sensitivity of fiber lightguides.The objective of the present work was to investigate the absorption spectra of unirradiated and neutron pre-irradiated fiber lightguides before and after additional γ irradiation.Experimental Results and Discussion. The objects of investigation were FVP-300 and -400 (quartz-quartz) fiber lightguides manufactured by the Polymicro Company. According to the data provided by the manufacturer, the concentration of OH groups in the core of fiber lightguides made of pure quartz glass is ~1000 ppm. These samples were irradiated in the VVR-SM nuclear reactor at the Institute of Nuclear Physics of the Academy of Sciences of the Republic of Uzbekistan up to neutron fluence 10 12 and 10 14 cm −2 (E > 0.1 MeV). The dose rate of the accompanying γ radiation, determined by using a KI dosimeter based on quartz glass, is 1250 R/sec. To determine the contribution of fast neutrons and the accompanying γ radiation of the reactor to the formation of defects, the absorption characteristics of unirradiated and pre-irradiated