The low-lying electronic transitions in a peroxy radical, =Si-O-O-., a fundamental radiation induced point defect in silicon dioxide, have been investigated by means of ab initio multiconfiguration perturbation theory calculations, CASPT2, and cluster models. The accuracy of the computed transition energies and intensities as predicted by the CASPT2 approach has been checked by studying the exited state properties of two molecular analogues, the HOO. and CH3OO. radicals, and of a well-characterized paramagnetic defect in silica, the nonbridging oxygen center, =Si-O-., for which unambiguous assignments exist. We found that the peroxy radical gives rise to two optical absorption bands, a very weak one at 0.7 eV and an intense one at 5.49 eV, in agreement with the experimental assignments of Radzig [V. A. Radzig, Chem. Phys. Reports 14, 1206 (1995)].