A heterometallic complex of cobalt(III) and bismuth(III), [Co(NH₃)₅NCS]₂[(Edta)Bi(μ-C₂O₄)Bi(Edta)] · 12H₂O, was synthesized and studied by X-ray diffraction analysis. The crystals are monoclinic, a = 12.792(3) Å, b = 9.792(2) Å, c = 22.945(5) Å, β = 92.92(3)°, P(calcd) = 1.970 g/cm³, space group P2₁/c, Z = 4. The complex has an isle structure composed of [(Edta)Bi(μ-C₂O₄)Bi(Edta)]^4- and [Co(NH₃)₅NCS]²⁺ complex ions and molecules of crystallization water. The Edta^4- ligand functions as a hexadentate chelating (2N + 40) agent, while the oxalate ion acts as a bridging ligand. The thiocyanate ion is coordinated to cobalt(III) through the N atom, while the cation interacts with the anion via the Bi-S contact (3.571 Å). Hence, the coordination polyhedron of Bi can be regarded as a tricapped trigonal prism. It was shown that thermolysis of the complex proceeds in several stages, viz., the deaquation, deamination, and pyrolysis of its ligands.