An X-ray photoelectron spectroscopy study of Mo/Al2O3 catalysts prepared via [(Mo2O4)-O-V(C2O4)(2)(H2O)(2)](2-) complexes showed that after heating the catalysts with hydrogen in the spectrometer chamber, the position of the Mo3d line shifted to higher values of binding energy. This shift is interpreted as oxidative addition of hydrogen to the surface Mo species. A similar phenomenon was observed for a CO treated catalyst. A temperature-programmed desorption study has shown that hydrogen is strongly bound to Mo and can only be removed from the catalysts at temperatures as high as 500degreesC.