This paper presents gamma-ray irradiation effects on OH-loss increase due to the chemical reaction of H2for GeO2-doped and GeO2. P2O5-doped fibers. When H2diffuses into irradiated fibers (γ-H2treatment), OH-loss increase occurs even at room temperature. Moreover, OH-loss increases due to dissolved H2in irradiated fibers are larger than those in unirradiated fibers at high temperatures. These OH-loss increases are attributed to irradiation-induced defects with which H2reacts to form OH ions. When H2-dissolved fibers are irradiated, the OH-loss increases are larger than those in the γ-H2-treated fibers at room temperature. These larger loss increases are thought to be caused by the reaction of the unstable radical and dissolved H2, which are excited by the irradiation.