Abstract: A process for purifying concentrates of molybdenum oxide containing a variety of naturally-occurring contaminating metals, such as potassium, copper, calcium, magnesium, iron, aluminum, lead, zinc, bismuth, and the like, as well as compounds thereof, by roasting the impure concentrate at an elevated temperature in an oxygen atmosphere to convert sub-oxides of molybdenum to molybdenum trioxide and contaminating metal molybdites to corresponding metal molybdates, which are extracted by a hot aqueous leaching of the concentrate and the solubilized molybdate ions are recovered. The aqueous leached concentrate, after separation from the aqueous leach solution, is leached with an ammoniacal leach solution for converting the molybdenum trioxide constituent therein into soluble ammonium molybdate compounds, and the resultant leach solution is removed from the remaining insoluble residue, which is discarded.