Octane number of synthetic gasolines, for example, from the Fischer-Tropsch process, is increased by heating with fuller’s earth, activated with acid or with small amounts of AlCl3, ZnCl2, FeCl3, or BF3. Silica gel or alumina may be employed in place of clay. Gasoline obtained by hydrogenation of CO, b. p. range 40°-200° C., was passed at 180°-200° C. over activated fuller’s earth at the rate of 400 cc. per hr. per kg.; octane no. was raised from 44 to 51.