Temperature programmed reduction (TPR) and X-ray diffraction (XRD) were performed for the characterization of supported MoO₃ catalysts. Silica supported MoO₃ (surface loading from 0.2 to 4.0 atoms Mo/㎚²) and alumina supported MoO₃ (surface loading from 0.6 to 12.0 atoms Mo/㎚²) were reduced from 400 to 1000 C. TPR results of physically mixed samples show that bulk MoO₃ is reduced in the first reduction stage. This is in contrast to the literature where the first reduction peak is attributed to dispersed and microcrystalline of MoO₃, while the second peak is attributed to bulk MoO₃. It is also noted that the area of the first TPR peak is close to half that of the second TPR peak. This implies that MoO₃ crystals are reduced over supports by a two step process as Mo^(6+) →Mo^(4+) →Mo metal. Finally, 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.