Oxygen vacancy (OV) and peroxy linkage (PX) pairs in silicon dioxide are investigated using ab initio density functional calculations. We find that the pair defects in which the peroxy linkage is located adjacent to the oxygen vacancy have an electron-trapped thermodynamic charge state level in the silicon band gap and that the geometrical structure of the electron-trapped level is the asymmetrical double Si-O-Si bond structure of the pair defect. (C) 2003 Elsevier Science B.V. All rights reserved.