© The Electrochemical Society. In this study, Cr-poisoning in a SOFC mixed conducting cathode was investigated. To examine the influence of the electrode reaction systematically, a model patterned thin film electrode was used. La0.6Sr0.4CoO3-δ was chosen as the model cathode material. The LSC model electrode was exposed to Cr-containing oxygen at 973 K under a cathodic polarization. The Cr content in the feed gas was controlled by setting the temperature of a Cr-source at 973 K or 773 K. The degradation of the electrode performance was observed under both conditions. Deposits observed on the electrode surface were SrCrO4 for the higher Cr-concentration in the feed gas. On the other hand, deposits were Sr-compounds without detectable amount of Cr for the lower Cr-concentration. These results may indicate that not only the Cr-deposition but also the decomposition of the electrode material could be considerable causes for the Cr-poisoning in mixed conducting cathodes depending on the Cr-concentration.