A. Llordes, K. Zalamova, S. Ricart, A. Palau, A. Pomar, T. Puig, A. Hardy, M. K. Van Bael and X. Obradors Chem. Mat. 22, 1686-1694 (2010) DOI: 10.1021/cm903080k A thorough analytical study on the thermal decomposition evolution of the metal-trifluoroacetate precursor toward high-performance YBa₂Cu₃O₇ superconducting films is presented. Evolved gas analysis (EGA), using Fourier transform infrared spectroscopy (FTIR) and mass spectrometry (MS), as well as X-ray diffraction (XRD), was performed to determine the complete chemical decomposition reaction of the metal-trifluoroacetate precursors. It is noteworthy that, contrary to what had been previously described, HF was not detected in the released gas. Moreover, we present new processing conditions that successfully reduced and even eliminated the undesirable porosity of the pyrolyzed films. Focused-ion-beam (FIB) studies demonstrated that the formation of pores was related to a fast escape of the released gas during precursor decomposition. The oxygen partial pressure was determined to be a key parameter to control both the kinetics and thermodynamics of the decomposition reaction and, hence, the porosity. This is of great importance because dense films are required to achieve high critical current densities in YBa₂Cu₃O₇ superconducting films.