Heteronuclear complexes in the series La[FexCo1-x(CN6)]. nH(2)O, with x = 0 similar to 1 were prepared. Some complexes consisted of a mixture of hexagonal and orthorhombic phases, due to the presence of different amounts of crystallization water molecules. The lattice constants estimated for the orthorhombic structures increased linearly with increasing Fe content, x. The formation of the perovskite-type oxides by the thermal decomposition of the complexes has been studied by simultaneous thermogravimetric and differential thermal analysis (TG/DTA), X-ray fluorescence (XRF), and X-ray diffraction (XRD). The thermal decomposition of the complexes caused the formation of a single perovskitic phase at different temperatures, depending on the Fe content. Orthorhombic LaFeO3 and hexagonal LaCoO3 were already obtained at about 600 degrees C from the decomposition of the corresponding complexes. For the complexes containing both Fe and Co, the elemental analysis showed that the Fe content was always lower than the prescribed ratios. The single perovskitic phase was formed with increasing temperature as the prescribed ratios of Fe and Co became more similar, the maximum being for x = 0.5 at 1000 degrees C, a single hexagonal phase was formed or x less than or equal to 0.35, and a single orthorhombic phase was formed for x greater than or equal to 0.44, both being the measured values.