Using photoelectron spectroscopy and first principles molecular orbital calculations, we report the first observation of an abrupt change in the electronic structure Of W4Om- (m less than or equal to 6) clusters at m = 5 which is well below the bulk stoichiometric composition of tungsten oxide (WO3). The signature of this onset is established from an anomalous increase in the vertical detachment energies (VIDE), adiabatic electron affinities (AEA), and the energy gaps between highest occupied (HOMO) and lowest unoccupied molecular orbitals (LUMO). These changes are also accompanied by the cleavage of 50% of the W-W bonds and localization of electron charge density in W4O5 cluster. (C) 2004 Elsevier B.V. All rights reserved.