Results of first-principles calculations on the structure and electronic structure of amorphous WO3 are presented. The effect of non-stoichiometry is investigated. In particular, we discuss the pairing of W5+ species in oxygen-deficient films resulting in deep in-gap states and its possible consequences for the electrochromic coloration efficiency. To this end, we make a connection with Raman experiments by calculating the vibrational density from molecular-dynamics simulation. For the W5+ -W5+ stretch we find a low frequency mode at similar to 200 cm(-1) which agrees well with the Raman data. We also estimate the stability of isolated W5+ species in stoichiometric and oxygen-deficient tungstentrioxide.