Abstract: High-purity MTiO.sub.3 (M=Sr and/or Ba)-type dielectric thin films with improved electric characteristics, particularly leakage currents and dielectric breakdown voltages, are prepared by MOCVD. Either or both a high-purity bis (.beta.-diketonato) Sr and Ba complexes, which each contain 1 ppm or less of each alkali metal and an alkaline earth metal as impurity metals, are used as the metal M supply sources. The high-purity volatile complexes are prepared by heat decomposition Sr or Ba nitrate (or acetate), which has been purified by a combination of recrystallization and ion-exchange chromatography, to contain 1 ppm or less of each alkali metal and alkaline earth metal as impurity metals, to thereby prepare high-purity SrO or BaO. The SrO or BaO is then reduced to high-purity metallic Sr or Ba by the thermit process, and then the metallic Sr or Ba is reacted with a .beta.-diketone to form the bis(.beta.-diketonato) complexes.