The possibility of preparing fine-particle BaZrO 3 by hydrothermal synthesis was examined. The samples prepared using three different starting mixtures were characterized by x-ray diffraction, thermogravimetry, and scanning and transmission electron microscopy techniques. By dissolving solid Ba(NO 3) 2 in a 0.25 M ZrO(NO 3) 2 solution (pH 1.5), nanocrystalline powder of the stable polymorph M-ZrO 2 was obtained (particle size d = 8-10 nm), independent of the Ba(NO 3) 2 : ZrO(NO 3) 2 ratio. Hydrothermal treatment of a ZrO(OH) 2 gel in neutral (pH 7.0) and ammonia (pH 11.0) media in the presence of Ba(NO 3) 2 yielded only the metastable phase T-ZrO 2 (d = 10-12 nm), whereas treatment under similar conditions with no Ba(NO 3) 2 in solution resulted in a mixture of M- and T-ZrO 2. The formation of microcrystalline BaZrO 3 (d = 2-5 μm) from a ZrO(OH) 2 gel was observed only in a high-pH Ba(OH) 2 solution (pH 13.0). Attempts to obtain BaZrO 3 by boiling an amorphous ZrO(OH) 2 gel at 378 K at atmospheric pressure in a high-pH Ba(OH) 2 solution (pH 13.0) for 45 min with the use of microwave heating were unsuccessful: the solid phase consisted of x-ray amorphous ZrO(OH) 2.