Thin-film growth of complex oxides including BaTiO3, SrTiO3, BaZrO3, SrZrO3, KTaO3, and KNbO3 were studied by the hydrothermal and the hydrothermal-electrochemical methods. Hydrothermal-electrochemical growth of ATiO(3) (A = Ba, Sr) thin films was investigated at temperatures from 100 degrees to 200 degrees C using a three-electrode cell. Current efficiency for the film growth was in the range from ca. 0.6% to 3.0%. Tracer experiments revealed that the ATiO(3) film grows at the film/substrate interface. AZrO(3) (A = Ba, Sr) thin films were also prepared on Zr metal substrates by the hydrothermal-electrochemical method. By applying a potential above ca. +2 V vs. Ag/AgCI to the Zr substrates, AZrO(3) thin films were formed uniformly. KMO3 (M Ta, Nb) thin films were prepared on Ta metal substrates by the hydrothermal method. Perovskite-type KTaO3 thin films were formed in 2.0 M KOH at 300 degrees C. Pyrochlore-type K2Ta2O6 thin films were formed at lower temperatures and lower KOH concentrations. Morphotropic phase changes were also revealed in the hydrothermal system KTaO3-KNbO3. (c) 2006 Springer Science + Business Media, Inc.