The heteroepitaxial growth of YBa2Cu3O7-x (YBCO) films prepared by the trifluoracetate TFA route was characterized by TEM/EELS, ν-Raman spectroscopy and X-Ray diffraction of specimens quenched from various temperatures. We find that after the pyrolysis, the film consists of a homogeneous, partly amorphous, nanocrystalline matrix of Ba 1-xYxF2 + x and CuO. Upon heating, such a precursor undergoes a strong phase segregation on a length scale of 100 nm. Simultaneously, the Ba1-xYxF2 + x solid solution is decomposed into BaF2 and Y2O3, and part of this Y2O3 eventually reacts with the CuO to give Y2Cu2O5. Our results make evident that the nucleation of YBCO takes place exclusively at the interface with the substrate, within the fluoride phase. The fluoride phase appears highly textured from the early stages of phase evolution, above 600 °C, and determines the orientation of the YBCO. The microstructural heterogeneity of the precursor film prior to the nucleation of YBCO strongly suggests that more than one reaction path may operate simultaneously. © 2006 IOP Publishing Ltd.