High temperature X-ray diffraction analysis is employed to study the reduction of three commercial complex ammonium molybdates marked as AM-l, AM-2 and AM-3 which are used as the starting material for the production of molybdenum metal. The compositions of AM-l, AM-2 and AM-3 are (MH4;)6Mo7024-4 H20 (small amount) + (NH4)Mo5O+ (NH.)2Mo40(small amount) + (3 -(NH4)Mo4O13(NH4):Mo40, + β -(NH4)Mo40, + (NH4)Mo40,-2 H6 (small amount) and (NH4(2MoO13 + β - (NH4)2Mo4O13+(NH4)2Mo4O13-2 H2O respectively. It is found that the thermal decomposition of ammonium molybdates into MoO3, proceeds prior to the reduction of MoO3 The intermediary products of the thermal decomposition, (NH4)Mo14041 and identified which arc more stable in hydrogen than in air. MoO.80) and Mo40 are also identified as the intermediary products during the reduction of MoO to MoO. The reduction reaction of Mo4Ois infered from the phase development of AM-l, AM-2 and AM-3 during the reduction course. The thermal decomposition of AM-2 and the subsequent reduction of MoO occur almost at the same time while those of AM-l or AM-3 take place at a larger range of temperatures, which is identical with what is observed in the practical production process.