Vanadium oxides, such as V2O3, VO2, and V2O5, have attracted considerable attention because of the fascinating physical properties of each oxidation state. On the other hand, precisely controlling the individual oxidation states is difficult due to the sensitivity of oxygen stoichiometry. This article reports that reactive oxygen annealing (ROA) can systematically change the oxidation state of the vanadium oxide films grown on a c‐Al2O3 substrate compared with typical annealing under O2 environments. Chemical, structural, electronic, and electrical analysis confirms the evolution of the vanadium oxide phases from V2O3 to V2O3/VO2, VO2, VO2/V2O5, and V2O5, showing that the ROA method can control and tune the oxidation state of the highly oxygen‐sensitive multivalent metal oxides.
