The electrode reaction mechanism was investigated at the porous La1-xSrxCoO3-d (LSC)/La0.8Sr0.2Ga0.8Mg0.2O3-y (LSGM) interface (x = 0.2, 0.3, 0.4). Direct current (dc) polarization curves showed a linear relationship between the overpotential and the current density at the lower cathodic polarization. The interface conductivity was adopted to examine the activity for oxygen reduction at the LSC/LSGM interface, which was measured both from the dc polarization curves and the alternating current impedance. The interface conductivity increased with Sr concentration in LSC and cathodic applied voltage. The higher Sr concentration of LSC showed the lower oxygen partial pressure dependence of the interface conductivity. The activation energy for the interface conductivity was almost the same for different Sr concentration of LSC (137-142 kJ mol(-1)). A similar reaction mechanism is assumed for the examined LSC. The interface conductivity has a relation with oxide ion diffusion in LSC, which can be the main factor to determine the cathode reaction rates under cathodic polarization.