The oxidation states of vanadium and tungsten were studied by XPS technology. Fig. 5 showed typical XPS spectra of W-doped VO2 sample (w: 2.5at.%). There were only four elements, which were carbon, vanadium, oxygen and tungsten could be detected, where the carbon was attributed to surface contamination (Fig. 5(a)). From Fig. 5(b), it could be found that the V2p3=2 peak of the particles was centred at 516.5 eV and O1s was centred at its standard value (530.0eV). The value for V2p3=2 peak was slightly higher than that for pure VO2 powder [40], but is in agreement with the value 516.3eV studied by Li [44] for V0.99W0.001O2, suggesting that the binding energy of V2p3=2 increased slightly after W-doping. Fig. 5(c) showed that the W4f peaks of the sample with binding energies of W4f7 and W4f5 centred at 35.4 and 37.5 eV respectively. According to the standard binding energy, that the oxidation state of tungsten ion in these powders is W6þ could be concluded [45].