Collision induced dissociation experiments of the alkoxo [W3S4(dmpe)(2)(O)(OCH2CH3)](+) tungsten (IV) cation reveal that aldehyde elimination is the dominant reaction pathway. Complementary deuterium labelling experiments give support to a hydrogen transfer mechanism, where the hydrogen atom exclusively originates from the alpha-position of the alkoxo ligand. On the basis of DFT calculations, two competitive mechanisms are proposed: one of them involving a proton transfer from the alpha-position of the alkoxo ligand to an oxygen atom of the vicinal W=O group; the other corresponding to a hydride transfer mechanism from the alpha-position of the alkoxo ligand to the geminate tungsten center. The calculated energy profiles show that the former is thermodynamically favoured and the second is kinetically favoured, with small energy differences between the two reaction paths; in consequence, both mechanisms compete under our experimental conditions. The proton transfer mechanism occurs through a seven-membered transition state structure while hydride transfer takes place through a four-center structure defined by the metal and the oxygen, carbon and hydrogen atoms of the ethoxo group. (C) 2011 Elsevier B. V. All rights reserved.