A resonating valence bond electron transfer mechanism of combining two O-2 molecules to form an O-4 molecule is presented. The predicted molecular states of the reaction path D-infinityh --> C-2v --> D-2h are supported by the present ab initio molecular orbital calculations. The CASPT2 BSSE calculations yield a stable diamagnetic D-2h O-4 molecule with a very weak chemical bond between the monomers, in good agreement with experiments. A low activation barrier energy of similar to26 cal/mol for the O-4 formation is found. (C) 2003 Elsevier Science B.V. All rights reserved.