First-principles total energy calculations are carried out to investigate the relative energetics of several neutral oxygen species in alpha-quartz and amorphous SiO2. We considered both atomic and molecular oxygen, as interstitial and network species. The interstitial O-2 molecule is found to be the most stable oxygen species, both in alpha-quartz and amorphous SiO2. The formation energy of the interstitial O-2 molecule shows a strong correlation with the size of the interstitial void, strongly decreasing for increasing cage sizes. (C) 2002 Elsevier Science Ltd. All rights reserved.