We describe a simple, non-destructive method, in situ thermal ellipsometry analysis (TEA), for understanding the different processes (decomposition of organics, crystallization, and sintering) occurring upon heating hybrid organic–inorganic films. According to these studies, a thermal treatment was tailored in order to obtain robust, nanocrystalline inorganic mesoporous 100–150 nm thick films with efficiently connected porosity surrounded by a crystalline inorganic network. Polymodal porous, nanocrystalline NiO/Gd-doped Ceria composites or Ni/Gd-doped Ceria films, interconnected network of open pores ranging from macro- to micro-pores, have been synthesized. The inorganic network is built from connected crystalline nanoparticles with mean diameters of 12 ± 3 nm, whose small size is still preserved even at 800 °C. We also show that the thermal ellipsometry analysis is readily extendable to MO/Gd-doped Ceria with M = Cu, Ni, Co, etc., therefore demonstrating the interest of this technique in understanding thermal phenomena in complex ceramic and composite systems. This is trivial for designing electrodes with efficient microstructure.