This paper describes the use of in-situ Fourier transform infrared spectroscopy to monitor the gas phase reactions of the formation of VO2 thin films from VO(acac)(2) under atmospheric pressure chemical vapour deposition conditions. In the absence of O-2, it is found that anhydride species may form, while there is also some evidence of ester species. In the presence of O-2, the spectra obtained are almost identical to those in the absence of O-2. However in this case, there is also some indication for the enhanced production of CO and the suppression of the formation of C-H species. A possible mechanism for the formation of VO2 is proposed, which involves the release of two C3H4 molecules and the decomposition of vanadyl (IV) acetylacetonate into VO(CH3COO)(2), which then further decomposes to yield (CH3CO)(2)O and VO2. However, while spectroscopic evidence for the formation of these species is presented, the mechanism proposed cannot be confirmed on the basis of these data alone. (C) 2007 Elsevier B.V. All rights reserved.