Mo L(3)-edge XANES and diffuse reflectance UV-vis spectra have been recorded for structural studies of Mo-Mg binary oxides which catalyze metathesis reactions of olefins after pretreatment with H-2 at 773 K. The XANES of reference compounds revealed that the local symmetry around the Mo ion as well as the valence state affects the spectral features. From the XANES of Mo-Mg binary oxides in oxidized/reduced states, it is concluded that MoO4 tetrahedra are the main component in the near-surface region for samples of x (Mo/(Mo + Mg)) less than or equal to 0.7 in the oxidized state. For samples in the reduced state, the tetrahedral species remain, but reduced Mo ions including MoO2 species are also formed, while the MoO2 phase is formed in the bulk phase at x = 0.5-0.8. In the latter samples, Mo ions are easily reduced by treatment with H-2. The active species for metathesis reactions relates to formation of the MoO2 phase not only in the bulk but also in the near-surface region.