An approach to produce molybdenum trioxide from spent hydrodesulfurization (HDS) catalyst, obtained from a petroleum refinery, is presented here. The spent catalyst was devolatilized at 600 ℃ so as to make it free from oils, organics and other volatile species. It was then roasted with sodium carbonate at a temperature of 850 ℃ for 30 min. The leaching efficiency for 20% soda roasted sample at 10% pulp density was 99.8%. From the solution molybdenum was precipitated out as ammonium molybdate at pH 1.0 with HCl and ammonium chloride. This ammonium molybdate was calcined at 750 ℃ to get MoO3. The product was characterized by XRD. Its purity was determined titrimetrically and by ICP-AES.