The electronic structure of polycrystalline monoclinic WO3 has 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 σ 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 π bonding states at the Fermi level has been also detected at these crystallites boundaries.