The oxygen reduction mechanism was investigated at the porous La1-xSrxCoO3-d cathode/ La0.8Sr0.2Ga0.8Mg0.2O2.8, electrolyte interface (x=0.2, 0.3, 0.4). The polarization resistance, measured from the impedance spectra, was compared in the samples of La1-xSrxCoO3-d as functions of x, temperatures, and applied DC voltages. The polarization resistance decreased with an increase of x values in La1-xSrxCoO3-d and with the applied cathodic voltage. The polarization resistance of the higher Sr-concentration in La1-xSrxCoO3-d showed the lower dependence on cathodic overpotential. The values of the activation energy of the interface conductivity (inverse of the polarization resistance) were similar for all La1-xSrxCoO3-d samples (127-143 kJ mol(-1)) at zero applied voltage (E = 0 V). However, under cathodic polarization, the activation energy decreased as the applied voltage became more negative, which indicates a change of the reaction mechanism under cathodic polarization. Under cathodic polarization, oxide ion diffusion in the bulk La1-xSrxCoO3-d can be one of the main factors determining the reaction rates.