The electronic structure of polycrystalline monoclinic WO3 was been investigated by means of scanning tunneling spectroscopy in ultrahigh vacuum. Current image tunneling spectroscopy maps show a clear and systematic increase of the conductance of the surface at the WO3 crystallite boundaries, with a corresponding decrease of the bandwidth. This effect is assigned to the occurrence of sigma bonded electronic states lying deep in the bulk band gap, and due to pairing of reduced W ions at intersection planes between adjacent crystallites. Significant presence of pi bonding states at the Fermi level has been also detected at these crystallites boundaries. (C) 2001 Elsevier Science B.V. All rights reserved.