Microstructure stability and electrical properties of Ni(O)–GDC (Ce0.9Gd0.1O2-δ) porous thin films were studied by both in situ X-ray diffraction and in situ impedance spectroscopy during the reduction of NiO into metallic Ni. The initial microstructure of the NiO/GDC porous thin film is composed of particles with diameter ≤10 nm delimitating a porous volume of 40%. The thin films microstructure is composed of two types of interpenetrated network, porous and inorganic. The different steps that take place during the reduction for Ni(O)–GDC porous thin films were highlighted and discussed based on the determination of their respective activation energy. The percolation threshold for metallic Ni appears when the Ni content reached a value of 30 vol %. Coarsening and coalescence of nickel particles occur at high temperature leading to a decrease in the total film conductivity with time. Finally, films containing 70 vol % of Ni keep a conductivity of 1 × 105 S/cm after heat treatment at 400 °C for 1000 min. This study shows that impedance spectroscopy is a useful tool to follow the reduction of NiO into Ni in GDC mesoporous film. It constitutes an aid for defining the experimental conditions (temperature and reducing time), keeping both good electrical and catalytic performances and for studying the evolution of the microstructure when films are maintained under H2 with time.