PURPOSE: To prepare high-quality molecular iodine from ammonium iodide in high yield, by oxidizing ammonium iodide in an aqueous medium containing a V compound and a weak acid.

CONSTITUTION: Ammonium iodide is oxidized with oxygen or air by using any of almost all V compounds as a catalyst in an aqueous medium containing the V compound and a weak acid such as phosphoric acid, ammonium dehydrogenphosphate, etc., to give molecular iodine. A weak acid obtained by hydrolyzing an ammonium salt of weak acid such as ammonium phosphate formed by the oxidation for the preparation of iodine is partially used as the weak acid. Iodine is obtained from ammonium iodide in high yield economically without preparing by-products such as nitrogen iodide, etc.