This paper reports on the stoichiometry determination of epitaxially grown vanadium monoxide (VOx) thin films on MgO(100) substrates. The epitaxial growth was confirmed by RHEED, LEED and XRD techniques. The oxygen content of VOx thin films, as a function of oxygen flux, was determined using Rutherford backscattering spectrometry. The O-18 isotope was used for film growth, in order to distinguish between the oxygen of film and substrate, The upper and lower stoichiometry limit found are consistent with the ones known for bulk material.