H-1 and C-13 Fourier transform-nuclear magnetic resonance (FT-NMR) and mass spectrometric (MS) techniques were used to prove the structures of precursors used in the sol-gel synthesis of the high-frequency dielectric Ba(Mg1/3Ta2/3)O-3. Various metal methoxyethoxides (MEO) of Ba, Ta, Mg-Ta and Ba-Mg-Ta systems were prepared in 2-methoxyethanol (MeOEtOH). The tantalum and the magnesium-tantalum precursors were determined to be Ta-2(MEO)(10) and MgTa2(MEO)(12), respectively. Similar spectroscopic studies for "Ba3MgTa2(MEO)(15)" indicated that the Ba-Mg-Ta alkoxide precursor has a polymer structure of (Ba,Mg,Ta)(MEO) complex and contains the bridging alkoxy groups between Ta and the alkaline earth metals.