Nanosized (10-30 nm particle size) hexagonal barium monoaluminate, having a high surface area of similar to30 m(2)/g, was synthesized by calcination of hydrothermally prepared precursors. The precursors were obtained by hydrolytic precipitation, using a mixed solution of barium and aluminum nitrates in the presence of aqueous urea at 180degreesC. Based on the results of XRD, FTIR, SEM/EDS, TEM, and TG-DTA studies, the most probable sequence of reactions leading to the formation of barium monoaluminate was (i) conversion of aqueous barium and aluminum nitrates in the presence of urea by hydrothermal precipitation to crystalline orthorhombic barium carbonate and boehmite, (ii) formation of an interim form consisting of amorphous fibrillar aluminum oxide(s) interlaced with crystalline barium carbonate in the calcination temperature range of similar to500degrees-800degreesC, (iii) initiation of formation of barium monoaluminate at 1000degreesC, and (iv) formation of a near monophase nanosized barium monoaluminate at 1200degreesC.