There is disclosed a process for preparing colloidal sols of antimony pentoxide in organic solvents which comprises the steps of:(a) reacting an alkali antimonate with 0.5 to 5 times of a monovalent or divalent inorganic acid in terms of stoichiometric ratio to form an antimony pentoxide gel,(b) after separating and washing the gel, aging the resulting wet cake at room temperature to 100.degree. C., and(c) peptizing the product in an organic solvent with Sb.sub.2 O.sub.5 being added, in terms of % by weight, 2 to 30% of an organic base and 0.2 to 20% of an organic acid as peptizing auxiliaries, and then removing water contained therein.