In situ high-energy X-ray diffraction measurements were conducted to elucidate the dynamics of barium zirconate (BaZrO3) nanoparticle formation in supercritical water. Time-resolved experiments of approximately milliseconds were achieved using a continuous-flow reactor and high-energy X-ray diffraction, which allows us to probe the sample in a stainless-steel tube. The size and structure of BaZrO3 in the early stage of crystallization under supercritical conditions (400 degrees C, 30 MPa) and on the order of milliseconds (46-66 ms) were successfully observed. An increase in the lattice parameter of BaZrO3 owing to a decrease in the number of defect sites in the time range was observed for the first time. A first-principles calculation confirmed the relationship between Ba defects and the lattice parameter and supported the formation mechanism of BaZrO3 in which nuclei, consisting of mainly zirconium, absorb the barium ion in supercritical water during crystallization. (C) 2015 Elsevier B.V. All rights reserved.