Heteronuclear complexes Ln[FexCo1-x(CN)(6)]4H(2)O (Ln = Pr-Yb) were synthesized and the crystal structures of thermally decomposed complexes were investigated. For Ln = Pr-Gd, only perovskite-type oxides (crystal system : orthorhombic) were formed even when the complexes decomposed at 800 degrees C. The lattice constants (a, b and c) of the oxides increased linearly with increasing Fe/Co ratio. On the other hand, for Ln = Dy-Yb, the Ln(2)O(3) phase was obtained as the major product, especially for lower Fe content. The single phase of the perovskite-type oxide could not be obtained even for x = 1, although the diffraction peaks due to the perovskite structure gradually increased with increasing Fe content.