Using vibrationally resolved ultraviolet photoelectron spectroscopy, we studied oxygen adsorption on W monomer anions. Three oxygen atoms are atomically bound in a WO3- compound, whereas in WO4- the fourth oxygen atom is attached to the oxygen, forming a di-oxygen species, implying that atom to molecule transition of O-2 takes place when the number of oxygen atoms attached to a W monomer anion exceeds three. Our results indicate that molecular adsorption of oxygen is preferred on electron-deficient metals, showing that the driving force of the atom to molecule transition for the chemisorption of diatomic molecules can be the variations of electronic structures of metal hosts. (C) 2004 American Institute of Physics.