In this study, we report on the thermal reduction of tungsten oxide studied with in situ photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). XPS measurements of the O1s core level of the annealed oxide film in vacuum show the appearance of a second state displaced in 1.1 eV towards higher binding energy. We propose that the shifted component is due to the difference in the charge state of oxygen anions and results from oxygen that is not bonded to metal. Simultaneous increase of the 0 1 6 ion yield measured with ToF-SIMS suggests that this higher binding energy component is related to atomic oxygen that is released from the oxide but remains in film. (c) 2006 Elsevier B.V. All rights reserved.