Abstract: The starting material is a vanadium-containing residue, which contains at least 5 weight percent carbon on an anhydrous basis. The residue is thermally treated in a furnace a) at temperatures from 400.degree. to 700.degree. C. under an oxidizing atmosphere and an O.sub.2 partial pressure of at least 10.sup.-4 bar, measured within the region which is occupied by the residue, and/or b) at temperatures from 500.degree. to 1300.degree. C. under an O.sub.2 partial pressure not in excess of 10.sup.-2 bar, measured within the region which is occupied by the residue. A solids mixture which contains at least 5 weight percent vanadium oxide is withdrawn from the furnace. A multiple-hearth furnace or a rotary kiln or a fluidized bed reactor containing a stationary or circulating fluidized bed may be used as a furnace for the thermal treatment.