Nanocrystalline La1-x Sr-x CoO3-delta (LSC) thin films with a nominal Sr content x = 0.5 were deposited on 3.5 mol% yttria-stabilized zirconia (YSZ) substrates by a low-temperature sol-gel process followed by a rapid thermal annealing procedure at temperatures up to 900 degrees C. The structural and chemical stability of the as-prepared nanocrystalline LSC and demixing effects within the thin film or at the LSC/YSZ interface were studied after long-time exposure at temperatures between 700 degrees C and 1,000 degrees C. The grain size and surface topography were analyzed by scanning electron microscopy. Transmission electron microscopy combined with selected-area electron diffraction, energy-dispersive X-ray spectrometry, and electron-spectroscopic imaging was applied for the investigation of the microstructure and the analysis of the local chemical composition and element distribution on the nanoscale. Chemical potential calculations, which were performed to assess the decomposition of LSC/YSZ as a function of temperature, show good agreement with the experimental results.