The influence of two Ti-precursors, TiO ₂ (anatase) and H ₂ TiO ₃ (β-titanic acid), on the purity and particle size of BaTiO ₃ powders prepared via hydrothermal synthesis is discussed. Amorphous H ₂ TiO ₃ was found to be an excellent Ti-precursor material and offers several advantages over crystalline anatase. Phase pure powders which have small particle sizes, ca. 40-80 nm and narrow particle size distributions can be prepared at 180 °C after 24 h using H ₂ TiO ₃ as a precursor material. Although the initial reaction is very fast, ca. 90% yield after 8-10 h, extended reaction periods at 180 °C are required in order to drive the reaction to completion. Lowering the reaction temperature from 180 to 85 °C does produce powders with even smaller particle sizes, however, very long reaction periods are required, e.g. >72 h, to ensure complete reaction. Raman spectra of as-prepared and heat treated (1000 °C) powders with average particle sizes as small as ca. 20-40 nm indicate asymmetry within the TiO ₆ octahedra of the BaTiO ₃ lattice. These results contradict the widely cited ‘critical’ particle size theory for the stabilisation of the cubic polymorph, at least for particle sizes greater than ca. 20-40 nm. As-prepared powders contain many defects, primarily in the form of lattice OH ^– ions. Preliminary ac impedance spectroscopy data on samples heat treated to remove lattice hydoxyl ions demonstrate these materials to be modest proton conductors.