Abstract: The formation and qualification of redox sites in transition metal oxides are always the active fields related to electronics, catalysis, sensors, and energy-storage units. In the present study, the temperature dependence of thermal reduction of MoO₃ was surveyed at the range of 350 ℃ to 750 ℃. Upon reduction, the formed redox species characterized by EPR spectroscopy are the Mo^V ion and superoxide anion radical (O₂^-) when the reduction was induced at the optimal temperature of 300-350 ℃. When heating-up from 350 ℃, the EPR signals started to decline in amplitude. The signals in the range of 400-450 ℃ decreased to half of that at 350 ℃, and then to zero at ~600 ℃. Further treatment at even higher temperature or prolonged heating time at 500 ℃ caused more reduction and more free electrons were released to the MoO₃ bulk, which results in a delocalized means similar to the anti-ferromagnetic coupling. These data herein are helpful to prepare and study the metal-oxide catalysts.