Both diffusion and reactions of interstitial oxygen atoms (O 0 ) in amorphous SiO 2 (a-SiO 2 ) were examined using oxygen-excess a-SiO 2 containing 18 Olabeled interstitial oxygen molecules (O 2 ) and exposed to F 2 laser light (hν = 7.9 eV). Both the F 2 laser photolysis of interstitial O 2 at 77 K and subsequent heat treatment at ≳200 °C give rise to oxygen exchange between residual interstitial O 2 and oxygen atoms in the a-SiO 2 network, and these temperatures are far lower than the temperature at which conventional thermal network-interstitial oxygen exchange in unirradiated a-SiO 2 occurs (≳700 °C). However, at the initial stage of the low-temperature F 2 laser photolysis, an efficient formation of interstitial ozone molecules (quantum yield ≳0.06) via nearly exchange-free diffusion of photogenerated interstitial O 0 is observed, and this reaction predominates over the network-interstitial oxygen exchange.