An interstitial hydroxyl radical (HO center dot) has been generated in bulk amorphous SiO2 (a-SiO2) loaded with interstitial H2O molecules and exposed to F-2 laser light (h nu = 7.9 eV, lambda = 157 nm) at 77 K. F-2 laser light dissociates an O-H bond of interstitial H2O into a pair of hydrogen atom (H-0) and HO center dot. The resultant H-0 disappears below 150 K, whereas HO center dot is detectable after thermal annealing at 200 K. The electron paramagnetic resonance (EPR) signal of the interstitial HO center dot recorded at 77 K is similar to that formed in amorphous ice, indicating that HO center dot is confined in an orthorhombic field by hydrogen bonding, probably with adjacent H2O molecules, silanol (SiOH) groups, and bridging oxygen atoms in the a-SiO2 network.