The growth of magnetron sputtered vanadium oxide thin films has been investigated by means of X-ray photoelectron spectroscopy and spectral transmittance in the region from 350 to 1100 nm, It is shown that film stoichiometry is dependent on film thickness as well as oxygen partial pressure. Beyond a critical thickness of similar to 250 nm, the films are completely stoichiometric V2O5 independent of the oxygen partial pressure. At low thickness, the films are a mixture of V2O3, VO2 and V2O5 in varying proportions depending on the. oxygen pressure, The presence of the non-stoichiometry has been correlated with changes in the spectral transmittance. It has also been shown that the mixed oxide phase is stable over a large regime of thicknesses and oxygen partial pressures during processing and could have very useful optical properties. A mechanism for the observed stoichiometry behaviour has also been proposed.