We investigate the re-oxidation of the reduced TiO2 (110) surface by adsorption of molecular oxygen using quantum-chemical, ab-initio periodic Hartree-Fock calculations. The absorption sites considered are anion defects and cation Ti(5f) positions. The results show evidence of strong charge transfer from the defect surface to the O-2 molecule. In agreement with experiment, we find that the most stable species is one in which molecular oxygen adsorbs at the defect sites as O-2(-). Adsorption of three O-2 molecules per vacancy site is thermodynamically favored compared to one or two adsorbed molecules. (C) 2002 Elsevier Science B.V. All rights reserved.