The electronic and molecular properties of interstitial O-3 in SiO2, have been theoretically studied by coupling the molecular cluster model to the density functional theory. We find that, on passing from the free species to the interstitial one, electronic and molecular structures Of O-3 are only slightly perturbed. Moreover, in agreement with the experimental assignment of Skuja et al., it is confirmed that the ubiquitous absorption band at 4.8 eV characterizing 7.9 eV photon-irradiated SiO2 samples includes a contribution due to excitations between O-3 based occupied and unoccupied MOs. (C) 2004 Elsevier B.V. All rights reserved.