The oxygen reduction kinetics was investigated at dense (La0.85Sr0.15)(0.9)MnO3 (LSM)-yttria-stabilized zirconia (YSZ) composite electrodes as a function of YSZ content of 20-40 wt% by using the potentiostatic current transient (PCT) technique combined with ac-impedance spectroscopy. From the discrete Fourier transformation analysis of impedance spectra, the individual reaction steps were successfully differentiated and the distribution of relaxation time for ion migration was quantitatively estimated for oxygen ion migration as well. The cathodic PCTs were characterized by decay and buildup transients, which corresponded to the oxygen reduction current at the three-phase boundaries (TPBs) I-TPB and the current at the LSM surface I-Vo(..) superimposed on the steady-state current at the TPBs I-TPB(st), respectively. As the YSZ content increased, the equilibrium exchange flux at the LSM surface j(o) decreased and the difference between the steady-state currents at the TPBs and at the LSM surface Delta I-st increased in absolute value. These results strongly indicate that when the YSZ content increases, the contribution of the facile ion migration to j(o) is suppressed by the prevailing contribution of the sluggish charge transfer reaction at the LSM surface and oxygen reduction at the TPBs predominantly contributes to the overall oxygen reduction than that reduction at the extended active sites. (c) 2008 The Electrochemical Society.