The interaction between the O-2 molecule and the neutral oxygen vacancy (F-S centers) located at the (0 0 1) MgO surface has been studied computationally, at the Hartree-Fock (HF) and density functional theory (DFT) levels, employing the molecular cluster approach. It has been found that, at short distances from the surface, a double electron transfer from the surface to the admolecules occurs, with the formation of peroxo O-2(2-) species. The process is highly thermodynamically favored with respect to the reactants (F-S + O-2). (C) 2004 Elsevier B.V. All rights reserved.