We have investigated the surface structure of monoclinic WO3 thin films on alpha-Al2O3(0001) substrates during thermal treatments in ultra high vacuum and oxygen exposure. For an annealing temperature of 450 degreesC, reflection high-energy electron diffraction shows the formation of a second WO3 phase with a pseudo cubic structure and the partial reduction to metallic tungsten clusters on the WO3 surface. Electron energy loss spectrometry confirms these changes of the surface structure. The reverse process, i.e. the oxidation of W, occurs if the samples are heated in air at 450 degreesC. Under oxygen exposure films in the monoclinic WO3 phase follow a semiconductor like behaviour with an activation energy of - 0.34 eV Reduced WO3 films exhibit a metal-like conductivity with an activation energy of approximately zero. The temperature-dependent conductivities as well as the conductivity effects that we found upon CO exposures to the WO3 thin films depend much on the thermal pre-treatments and the associated surface structures. CO exposure leads to an increase of the conductivity; this acceptor-type interaction of CO with samples that have been annealed at 450 degreesC is explained by a dissociation into carbon and oxygen, the latter interacting with oxygen vacancies. (C) 2004 Published by Elsevier B.V