A procedure to produce stable hydroperoxy radicals (HO2.) in bulk amorphous SiO2 (a-SiO2) has been developed. Oxygen molecules incorporated in the interstitial voids in a-SiO2 react with mobile hydrogen atoms (H-0) generated by the photolysis of silanol (SiOH) groups with F-2-laser light (lambda = 157 nm, hv = 7.9 eV), resulting in the efficient creation of interstitial HO2.. The high yield of HO2. suggests that the collisions of the reaction intermediate with the void wall play an important role in dissipating the excess energy of the intermediate instead of the triple collision observed in the gas phase reaction. The resultant HO2. is thermally stable up to 100 degrees C.