The oxygen reduction reaction on dense (La0.85Sr0.15)(0.9)MnO3-yttria-stabilized zirconia composite electrodes with various thicknesses was investigated as a function of the amount of secondary phase by analyses of the ac-impedance spectra and cathodic current transients. From the analyses of X-ray diffraction patterns, scanning electron microscopic images, and energy-dispersive spectroscopy mappings, it was found that the sintered electrode showed a dense structure and the amount of secondary phase, La2Zr2O7, on the electrode surface increased with increasing sintering time. From the analyses of the ac-impedance spectra and cathodic current transients, it was recognized that overall oxygen reduction reaction on the dense composite electrode proceeded under the mixed migration and charge-transfer control. Furthermore, based upon the fact that the ratio of the ion migration resistance R-i to the charge-transfer resistance R-ct decreases with decreasing thickness of the electrode and increasing amount of the secondary phase, it was conceivable that the charge-transfer kinetics and migration reaction rate were predominantly impeded by the secondary phase and the electrode thickness, respectively. From the above results, it is concluded that which reaction mainly contributes to overall oxygen reduction is crucially determined by the electrode thickness as well as the amount of the secondary phase. (c) 2007 The Electrochemical Society.