The local structure around Mo atoms in Mo-Mg binary oxides has been investigated by X-ray absorption spectroscopy at Mo and Mg K edges. XANES and EXAFS spectra show that the binary oxides have a tetrahedral MoO4 unit in the samples with a Mo/(Mo + Mg) ratio of less than 0.5 and an octahedral MoO6 unit in the samples with a higher ratio. By H-2 treatment at 773 K, a molybdena-rich binary oxide was easily reduced to form the MoO2 phase, which was not detected by XRD but was by EXAFS. It is concluded that the reduced oxide contains low valent Mo ions in the MoO2 phase and the ions work as an active center for propene metathesis.