The oxidation mechanisms and the numerous phase transitions undergone by VO2 thin films deposited on SiO2/Si and Al2O3 substrates when heated from room temperature (R.T.) up to 550 degrees C in air are investigated by Raman and X-ray photoelectron spectroscopy. The results show that the films undergo several intermediate phase transitions between the initial VO2 monoclinic phase at R.T. and the final V2O5 phase at 550 degrees C. The information about these intermediate phase transitions is scarce and their identification is important since they are often found during the synthesis of vanadium dioxide films. Significant changes in the film conductivity have also been observed to occur associated to the phase transitions. In this work, current and resistance measurements performed on the surface of the films are implemented in parallel with the Raman measurements to correlate the different phases with the conductivity of the films. A model to explain the oxidation mechanisms and phenomena occurring during the oxidation of the films is proposed. Peak frequencies, full-width half-maxima, binding energies and oxidation states from the Raman and X-ray photoelectron spectroscopy experiments are reported and analyzed for all the phases encountered in VO2 films prepared on SiO2/Si and Al2O3 substrates. (C) 2017 Elsevier B.V. All rights reserved.