Effects of the chloride ions on the synthesis of tetragonal BaTiO3 in the microwave-hydrothermal (MH) and conventional hydrothermal (CH) processes were investigated. Such effects were found to be dependent of the synthesis method and temperature. In the MH process performed at 240 degrees C, tetragonal BaTiO3 powders of c/a ratios larger than 1.009 were readily prepared whether the chloride ions were present or not. At the lower temperature of 220 degrees C, however, the c/a ratio in the MH BaTiO3 was only 1.0061 in the absence of the chloride ions but increased to 1.0084 aided by the chloride ions. In the CH process, the chloride ions were observed to play an invariably important role in the preparation of tetragonal BaTiO3. In contrast, the chloride ions appeared to have less influence on the particle size of the hydrothermal BaTiO3 in a relatively short period of time. (c) 2006 Elsevier B.V All rights reserved.