A low-temperature hydrothermal reaction scheme has been developed to produce pure, ultrafine, uniform-sized, nanocrystalline barium titanate (BaTiO3) microspheres from two inorganic precursors: synthesized titania microspheres and barium hydroxide solutions. The size and morphology of titania (TiO2) microspheres were controlled using isopropanol to fine-tune the dielectric constant of the isopropanol-water mixed solvent system. Monodispersed titania microspheres approximately 0.1-1 mu m in diameter were successfully synthesized for the further conversion to barium titanate. Barium titanate and titania microspheres were characterized by scanning electron microscopy (SEM) and room-temperature X-ray diffraction (RTXRD). High-temperature XRD (HTXRD) was also utilized for in situ study of the phase transformations and changes of crystallite size with calcination temperatures. The titania microspheres were predominant in the anatase (plus some brookite) phase at room temperature and were converted to the rutile phase when the calcination temperature was increased from 650 degrees C to 900 degrees C. Monodispersed barium titanate microspheres were successfully synthesized from optimized titania via a hydrothermal reaction (less than or equal to 100 degrees C) in barium hydroxide solutions. The size and morphology of the barium titanate particles remained the same as the precursor titania particles, indicating a "shrinking-core" diffusion-reaction mechanism. Barium carbonate in the form of witherite was also found along with the formation of barium titanate, especially under conditions with higher Ba/Ti ratios, but a formic acid washing procedure effectively removed this impurity phase from the barium titanate samples. The as-prepared barium titanate was in the cubic nanocrystalline form and did not change when the temperature was increased from room temperature to as high as 750 degrees C. The cubic phase was also stable at high temperatures for over 5 h.