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Energy dispersive spectrometry line scan and ac impedance spectroscopy were used in this study to investigate the chemical reactions between two cathode materials, La0.84Sr0.16MnO3 (LSM), La0.5Sr0.5CoO3-δ (LSC), and the electrolyte La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM). Significant interdiffusions of Co into LSGM and Ga into LSC were found at an LSC/LSGM interface even at relatively low fabrication temperatures. In contrast, only small interdiffusions of Mn into LSGM and Ga into LSM were detected at the LSM/LSGM interface even though it was fired at 1470°C. The ac impedance spectra of the electrolyte LSGM with LSM, LSC, and Pt electrodes indicate a grain‐boundary contribution to the total conductivity in the intermediate frequency range and a diffusion‐controlled impedance in the low‐frequency range. Irrespective of chemical reactions and a larger thermal expansion coefficient, LSC has the lowest dc resistance of all three electrodes investigated. Considering both the small interdiffusion reactions between LSM and LSGM and their similar thermal expansion coefficients, LSM could be an appropriate cathode material for LSGM‐based fuel cells.