Abstract: A process for purifying an impure molybdenum oxide concentrate feed material to remove therefrom the predominant portion of contaminating metals present by autoclaving at an elevated temperature under an oxygen partial pressure of at least about 100 psi, a slurry of the particulated feed material in an aqueous acid solution containing a mineral acid for a period of time sufficient to convert the predominant portion of molybdenum compounds to molybdenum trioxide and to convert the contaminating metal compounds present to aqueous acid soluble compounds. The autoclaved slurry is thereafter filtered and the solids are subjected to an aqueous alkali hydroxide leach treatment to extract the molybdenum trioxide constituent therein in the form of alkali metal molybdate compounds, which subsequently are recovered from the leach liquor in the form of a high purity alkali metal molybdate product.