We report a spectroscopic characterisation of MoO3, WO3 and a MoO3-WO3 mixed oxide thin films deposited on alumina and silicon substrates. Absorbance FT-IR and diffuse reflectance UV-Vis-NIR spectra were recorded after treatments in vacuum and after interaction with O-2, NO2/O-2, CO/O-2 or pure CO at increasing temperatures up to 673 K. For all the films, reducing treatments (vacuum, CO or CO/O-2) cause the increase of a variety of broad absorptions both in the Vis-NIR and medium IR regions. These absorptions decrease in intensity after contact with oxidising atmospheres (O-2 or NO2/O-2), so that they are all assignable to electronic transitions due to the presence of a variety of donor levels related to oxygen defects. (c) 2005 Elsevier B.V. All rights reserved.