A novel oxalate-based complex of the formula {Ba-2(H2O)(5)[NbO(C2O4)(3)]HC2O4}center dot H2O (1) was prepared from an aqueous solution containing the [NbO(C2O4)(3)](3-) and Ba2+ entities in the molar ratio 1:2, and characterized by X-ray single-crystal diffraction, IR spectroscopy, and thermal analysis. The crystal packing of 1 reveals a complex three-dimensional (3D) network: the Nb polyhedron is connected to eight neighboring Ba polyhedra through the oxalate ligands and the oxo-oxygen group, whereas the Ba polyhedra share edges and vertices. The ability of compound 1 to act as a single-source precursor for the formation of bimetallic oxides was investigated by the thermal analysis (TGA and DSC) and X-ray powder diffraction. Thermal processing of 1 resulted in the formation of mixed-metal oxide phases, Ba4Nb2O9 and Ba5Nb4O15. Three stable polymorphs of Ba4Nb2O9 were isolated: the known, hexagonal alpha- and orthorhombic gamma-Ba4Nb2O9, and another one, not previously reported, hexagonal delta-Ba4Nb2O9 polymorph. The new, delta-Ba4Nb2O9 polymorph has the 6H-perovskite structure (space group P6(3)/m), in which the Nb2O98- face-sharing octahedral dimers are interconnected via corners to the regular BaO610- octahedra. Formation of the mixed-metal oxides takes place at different temperatures: the Ba5Nb4O15 oxide occurred at similar to 700 degrees C, as the major crystalline oxide phase; by heating the sample up to 1135 degrees C, the alpha-Ba4Nb2O9 form was obtained, whereas the heating at 1175 degrees C caused the crystallization of two polymorphs, gamma-Ba4Nb2O9 and delta-Ba4Nb2O9. Special focus was set on the electrical properties of the prepared mixed Ba-II-Nb-V oxides obtained by this molecular pathway in a single-step preparation.