Using a density functional approach, we determine the relative stability of various oxygen species in alpha -quartz. We considered interstitial species in the form of atomic and molecular oxygen, as well as network species, such as peroxyl and ozonyl link-ages. The interstitial molecular oxygen species is found to be the most stable one. The ozonyl link-age lies higher in energy by 1.0 eV. Dissociation of the interstitial O-2 molecule into two peroxyl linkages or into two interstitial oxygen species occurs at even higher energies (1.6 and 3.2 eV respectively). These results favor a long-range diffusion mechanism based on the interstitial O-2 molecule. (C) 2001 Elsevier Science B.V. All rights reserved.