The formation of BaTiO3 thin films from (110) TiO2 rutile single crystals and BaCO3 was investigated experimentally by solid-solid and gas-solid reactions in vacuum. X-ray diffraction revealed the formation of an intermediate Ba2TiO4 phase before BaTiO3 is formed. According to our calculations the formation of BaTiO4 is associated with a maximum decrease in the Gibbs energy at a CO2 pressure lower than 10(-4) mbar. Reactions at 600-900 degrees C showed different processes to occur in the solid-solid and gas-solid reactions. The observations are interpreted in terms of the different mass transport mechanisms involved. The results shed new light on the phase sequence during BaTiO3 formation; in particular a dissociation of BaCO3 prior to its participation in the reaction has become rather unlikely. (c) 2006 Elsevier B.V. All rights reserved.