In this paper, we investigate the diffusion and reactivity at the interface between the electrolyte (YSZ) and the cathode (LSM) of planar SOFC single cells by Analytical and High Resolution Transmission Microscopy. Cells were obtained via an aqueous tape-casting process allowing casting the three layers (cathode, electrolyte, anode [Ni-YSZ]) in a single operation. To allow the cell to function at intermediate temperatures (750-850 degrees C), the final electrolyte thickness after co-sintering at 1350 degrees C must range between 40 mu m and 50 mu m. As-sintered cells as well as cells that have been operated at 850 degrees C have been considered here; it is shown that the electrical performances were one order of magnitude less than expected and that they deteriorated quickly under operating conditions. In order to explain this behavior, we carried out analytical transmission electron microcopy. To obtain precisely located chemical and structural information, we used the "H-Shape" as well as the Lift Out FIB (Focused Ion Beam) techniques to extract 5 x 10 x 0.1 mu m TEM samples; additional High Resolution characterization was carried out at interfaces between LSM and YSZ grains on standard ion-milled samples. We showed that the co-sintering temperature (1350 degrees C) was responsible for some diffusion of manganese through the electrolyte and the cathode, leading then to the rise of a significant electronic conduction and to the drop off of the ionic conductivity, and accounts for the germination and growth of the resistive pyrochlore phase La2Zr2O7. Operating the cell at 850 degrees C do not aggravate these phenomena, but rather alters the anode microstructure.< Leer menos