The hydrothermal crystallization of barium titanate, BaTiO3, has been studied in situ by time-resolved powder neutron diffraction methods using the recently developed Oxford/ISIS hydrothermal cell. This technique has allowed the formation of the ferroelectric ceramic to be followed in a noninvasive manner in real time and under genuine reaction conditions. In a first set of experiments, Ba(OD)(2). 8D(2)O was reacted with two different titanium sources, either crystalline TiO2 (anatase) or amorphous TiO2-H2O in D2O, at 100-140 degreesC and the reaction studied using the POLARIS time-of-flight neutron powder diffractometer, at the ISIS Facility. In a second series of experiments, the reaction between barium chloride and crystalline TiO2 (anatase) in NaOD/D2O was studied at temperatures between 100 and 200 degreesC and at different deuterioxide concentrations using the constant-wavelength D2O neutron powder diffractometer at the Institut Laue Langevin. Quantitative growth and decay curves were determined from analysis of the integrated intensities of Bragg reflections of starting materials and product phases. In both sets of experiments the rapid dissolution of the barium source was observed, followed by dissolution of the titanium source before the onset of crystallization of barium titanate. Using a nucleation-growth model we are able to simulate the growth curve of barium titanate at three temperatures. Our results indicate the predominance of a homogeneous dissolution-precipitation mechanism for the hydrothermal formation of barium titanate, rather than other possible mechanisms that have been discussed in the literature. Analysis of the line widths of the Bragg reflections in the neutron diffraction data indicates that the particle size of the BaTiO3 product phase prepared from the amorphous TiO2-H2O is smaller than that prepared from crystalline TiO2 (anatase).