Abstract: A method of preparing antimony pentoxide from solids containing antimony sulfides is described. The method comprises the steps of(a) leaching the antimony values from the solids with an aqueous potassium hydroxide solution to form an antimony-containing solution and a gangue,(b) oxidizing the antimony present in the leaching solution to a pentavalent antimony,(c) recovering the oxidized solution, and(d) adding an inorganic acid to the solution to reduce the pH of the solution to about 5-7 whereby antimony pentoxide is precipitated from the solution. The gangue may be separated from the leach solution either prior to or after the oxidizing step.In a preferred embodiment, the oxidation of the solution is conducted in the presence of a catalyst which comprises a water-soluble copper salt, a quinone, a hydroquinone or mixtures of one or more of these.