Temperature programmed reduction(TPR) and X-ray diffraction(XRD) were performed for the characterization of supported MoO₃catalysts. Silica supported MoO₃(surface loading from 0.2 to 4.0 atoms Mo/nm²) and alumina supported MoO₃(surface loading from 0.6 to 12.0 atoms Mo/nm₂) were reduced from 400 to 1000℃. TPR results showed that MoO₃ crystals are reduced over both supports by a two step process as Mo⁶⁺ →Mo⁴⁺→ Mo metal. The reduction mechanism over silica can be described as a simultaneous, parallel pathway for both types of species(polymolybdate and crystalline MoO₃) And well dispersed polymolybdate(peak maximum at 490℃) is reduced more easily than bulk MoO₃(peak maximum at 540) over alumina.