A sol having high oxidation purity and containing diantimony pentoxide particles having a primary particle size of from 2 to 50 nm and a Sb.sub.2O.sub.3/Sb.sub.2O.sub.5 weight ratio of at most 4 wt % is prepared by reacting a diantimony trioxide with an aqueous hydrogen peroxide solution in the presence of tunastate or molybdate.