Transmission electron microscopy was used to investigate the local chemistry and microstructure of the active cathodes in solid oxide fuel cells operated for 500 h (at 700 and 800 degrees C) under different electrochemical conditions while exposed to a chromia-forming stainless steel. Several distinct microstructural changes were observed owing to chromium interactions with the (La,Sr)MnO3 (LSM)-yttria stabilized zirconia (YSZ) cathode; the nature and magnitude of which depended on the temperature, electrochemical load, and physical location. Nanosized particles of (Cr,Mn)(3)O-4 and Cr2O3 were observed on the surface of the YSZ, regardless of the overall extent of degradation; the quantity increased with decreasing temperature and increasing current. The results indicate that Mn species facilitate the formation of a stable Cr-Mn-O nuclei on the YSZ, on which further growth occurs, including growth of Cr2O3. In cases of severe performance degradation, LSM decomposes completely, which does not appear to be strongly correlated with the nanoparticles on the YSZ, but results from a more destructive mechanism. Following this decomposition, severe pore filling of Cr-containing species occurs. The amount of microstructural degradation was largest near the cathode/electrolyte interface directly beneath the interconnect-cathode contact channels. These results indicate that two distinct mechanisms of degradation occur, with the electrochemical decomposition of (La,Sr)MnO3 as the primary cause for severe performance degradation. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3515073] All rights reserved.