Oxidation states of cations in a perovskite structure play an important role for conductivity in solid oxide electrochemical cells. For bulk materials oxidation states can be calculated for specific conditions, but is not well understood for micro−/nanostructured materials and at interfaces between materials. We present a fundamental study of interfaces between La0.6Sr0.4CoO3-δ (LSC) and yttria-stabilized zirconia (YSZ) in symmetric model solid oxide electrochemical cells. Nanoscale morphology as well as the chemical state of the LSC are investigated by scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). The experiments were performed in situ at temperatures up to 600 °C in high vacuum (ca. 10−7 mbar) and in 2 mbar oxygen. The measured LSC Co oxidation at room temperature is lower than that expected for bulk LSC, indicating a high oxygen vacancy density and possibly high ionic conductivity. However, the Co oxidation state increases with increasing temperature, both in oxygen and in vacuum. The results shows that the Co oxidation state approach that expected for bulk LSC for typical solid oxide fuel or electrolysis cell operation temperatures.