The mechanisms of photorefractivity in germanosilicate fibers are considered. Evidence is presented that high intensity irradiation significantly changes the state of the glass network in the irradiated core. The large changes of refractive index (>>10-4) are attributed to these structural changes. The possible roles of stress relief, nanocavitation and spinodal decomposition are discussed.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.