Abstract: A method for producing purified MoO.sub.3 from MoS.sub.2. MoS.sub.2 is initially oxidized to generate an impure MoO.sub.3 product containing metallic contaminants and molybdenum sub-oxides. This product is then slurried with a primary water supply to yield a first slurry in which part of the contaminants are dissolved. Preferred slurry temperatures are 25.degree.-97.degree. C. The resulting solid intermediate MoO.sub.3 product is removed from the slurry leaving the dissolved contaminants. Next, the intermediate MoO.sub.3 product is slurried with a secondary water supply to yield a second slurry in which the remaining contaminants are dissolved. Second slurry temperatures of 150.degree.-250.degree. C. are employed in the presence of an oxygen-containing gas. These conditions oxidize molybdenum sub-oxides to yield MoO.sub.3. The resulting purified solid MoO.sub.3 product is then removed from the second slurry. This process is highly efficient and avoids using liquid reagents other than water (including acids).