Abstract: Molybdenum trioxide containing potassium is heated to a temperature sufficiently high (i.e. at least about 525.degree. C.) to render the potassium soluble in hot water, the heated trioxide is quenched to less than about 400.degree. C. at a rate of at least about 30.degree. C. per minute, the quenched trioxide is leached with hot water (between about 50.degree. C. and about 100.degree. C.) to leach the solubilized potassium and after liquid-solids separation the molybdenum trioxide is dissolved in ammonium hydroxide to form ammonium molybdates, which can be recovered from solution and then can be calcined to chemical grade molybdic oxide. The process is advantageously conducted in conjunction with the roasting of molybdenite.