A simple route toward the synthesis of mesostructured composite La(0.7)Sr(0.3)Co(0.2)Fe(0.8)O(3)x/yGd-CeO2 (LSCFx/ yGDC) (with x and y representing the volumetric% of LSCF and GDC in the composite films) thin films exhibiting both high electrical conductivities and optimized bicontinuous pore-solid network is reported. Because of the size of the pore wall, the mesostructured composite film is crystalline with particle size in the nanometer range even after a heat-treatment at 700 degrees C in air. Crystallization of the film was studied by in situ impedance spectroscopy. This technique allows us to define the heattreatment, the LSCF content in the composite film and the thickness of the film to achieve good electrical conductivity. The highest electrical conductivity of 250 S cm(-1) was found for the pure, mesoporous LSCF film at 700 degrees C.