Heteronuclear Bi1-xLaxFe(CN)6.nH2O complexes were synthesised, and their crystal structures and thermal decomposition process were investigated by XRD, TGA, AES with SEM, and TEM. The crystal system of the complexes was both orthorhombic (n = 4 for x = 0-0.7) and hexagonal (n = 5 for x = 1.0). Their mixture was confirmed for x = 0.8 and 0.9. The lattice parameters for the orthorhombic phase increased with increasing value of x for the complexes. The single phase of trimetallic perovskite-type Bi1-xLaxFeO3 was obtained by thermal decomposition at low temperature. The crystal system was hexagonal for BiFeO3 (x = 0) and orthorhombic for x = 0.1-1.0. In the case of the decomposed perovskite sample, the lattice parameters decreased with increasing x values for Bi1-xLaxFeO3. The particle size was about 30 nm for Bi0.2La0.8FeO3 obtained by thermal decomposition at 500 C and it grew with an increase in decomposition temperature. For the Bi0.5La0.5FeO3, AES showed that the elemental distributions of Bi, La, and Fe on the surface were very homogeneous for the sample decomposed at 700 C. 23 refs.