The effect of external additives on the synthesis of fumed TiO2 with a high rutile structure content was studied. The focus of this investigation was on the external additive species, agglomerates of the fumed TiO2 before and after the thermal treatment. The transformation ratio from the anatase to rutile structure of the thermally-treated fumed TiO2 was investigated as a function of the fumed TiO2 agglomerate before the thermal treatment. Small agglomerated powders resulted in a decrease of transformation temperature. Two novel results were obtained in this investigation. One was a new fumed TiO2 with a 100% rutile structure having an excellent dispersibility being successfully synthesized by the thermal treatment of AEROXIDE (R) TiO2 P 25 with a small portion of AEROSIL (R) R 972. The other was the remarkable acceleration of the transformation from the anatase to rutile structure and grain growth/sintering of the thermally-treated fumed TiO2 observed at a relatively low thermal treatment temperature by the oxidation reaction of calcium stearate as an external additive. (C) 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.