Among (La_0.8Sr_0.2)_0.95MnO_3-δ (LSM) based cathodes LSM-(Er_0.2Bi_0.8)₂O₃ (LSM-ESB) composite shows promising performance and is one of potential cathodes for low temperature solid oxide fuel cells (LT-SOFCs). However, the high conductivity ESB exhibits oxygen-ordering at temperatures ≤600°C, resulting in an ordered super-lattice with low oxygen mobility. Here the electrochemical performance of LSM-ESB was investigated using electrochemical impedance spectroscopy (EIS) as a function of temperature, oxygen partial pressure (pO₂), bias, and time to determine possible electrochemical reaction steps and elucidate the oxygen reduction reaction (ORR) mechanism. LSM-ESB shows a strong pO₂ dependence and above 650°C the non-stoichiometry of LSM plays an important role in oxygen transport between solid phases under biased conditions. In addition, effects of ion conductivity of electrolyte on the performance of LSM-ESB are determined. Also, the high temperature reactivity between LSM and ESB solid phases is studied and the possible degradation mechanism is proposed. Our results provide insights in the oxygen transport on LSM-ESB through the comprehensive study.