X-Ray photoelectron spectroscopy (XPS or ESCA) was applied to the study of molybdenum oxides and supported molybdenum oxide catalysts. It was shown that the reduction behavior of supported oxide is substantially different from that of unsupported MoO 3. Reduction of pure MoO 3 at 400 °C, in a stream of hydrogen, for 2 hr, gave a nearly metallic species, while MoO 3 γ- Al 2O 3 was not reduced below Mo 4+. The greater extent of reduction of MoO 3 SiO 2 compared to MoO 3 γ- Al 2O 3 indicated a considerably weaker interaction between MoO 3 and SiO 2 than between MoO 3 and γ-Al 2O 3. Both α-CoMoO 4 (green) and β-CoMoO 4 (violet), upon treatment at 400 °C in flowing hydrogen, gave nearly metallic Mo and some metallic Co. On the basis of this behavior the presence of a cobalt molybdate phase similar to bulk CoMoO 4 on the surface of cobalt-molybdenum-alumina catalysts was excluded. The importance of a suitable reference level when considering chemical shifts of insulating samples was pointed out.