The early stage of barium titanate (BaTiO3) nanoparticle formation is investigated by in situ X-ray diffraction (XRD) and X-ray absorption near-edge structure (XANES) using synchrotron radiation. BaTiO3 nanoparticles are synthesized via dissolution of barium hydroxide octahydrate and hydrolysis of titanium (IV) isopropoxide in isopropanol. In the course of raising the temperature of the alkoxide-hydroxide mixture solution to 80 degrees C, in situ synchrotron XRD reveals that BaTiO3 nanocrystals smaller than 6 nm begin to nucleate at 50 degrees C without intermediate TiO2 anatase formation, and Ti K edge absorption spectra also confirm the formation of corner-sharing TiO6 octahedra at 60 degrees C. The average size of BaTiO3 precipitates increases to about 7.5 nm at 80 degrees C. The synthesized nanopowders show an anomalously high tetragonality according to the Rietveld refinement of synchrotron XRD data.