The long-term stability was investigated at the porous La0.6Sr0.4CoO3-d cathode/La0.8Sr0.2Ga0.8Mg0.2O2.8 electrolyte interface by AC impedance under cathodic polarization at 800 degreesC. The interface resistance at La0.6Sr0.4CoO3-d/La0.8Sr0.2Ga0.8Mg0.2O2.8 increased with an operation duration time under a cathodic polarization of -0.033 V (applied voltage of -0.3 V). However, the ohmic resistance of La0.8Sr0.2Ga0.8Mg0.2O2.8 decreased with prolonged duration time. After long-term operation test (for about 700 h), no significant reaction layer could be found at the La0.6Sr0.4CoO3-d/La0.8Sr0.2Ga0.8Mg0.2O2.8 interface by SEM/EDX analysis. Small amounts of Co were observed in the electrolyte side, which can affect the ionic resistance of La0.8Sr0.2Ga0.8Mg0.2O2.8.