The oxygen reduction kinetics was investigated at the porous (La1-xSrx)(1-y)MnO3 (LSM)-yttria-stabilized zirconia (YSZ) composite electrodes with various YSZ grain sizes by employing ac impedance spectroscopy and the potentiostatic current transient (PCT) technique. For this purpose, LSM-YSZ composite electrodes were coated on a YSZ pellet by sintering at 1473 K in air. The grain size of YSZ was controlled by varying the initial particle size of YSZ. From analyses of the ac impedance spectra and the cathodic PCTs measured on the electrodes as functions of temperature and ionic conductivity, it was first recognized that the overall cathodic reaction is mainly controlled by the oxygen-exchange reaction coupled with ion migration at low operating temperatures below 1073 K. Second, from the discrete Fourier transformation analysis of impedance spectra, it was confirmed that the distribution of relaxation times for oxygen-ion migration becomes wider with increasing YSZ grain size in the range from 0.6 to 4.8 mu m due to the larger effective migration length l(m). Finally, from the theoretical and experimental analyses of the cathodic PCTs based upon the equivalent circuit in consideration of the relaxation time distribution for ion migration, it is suggested that as the YSZ grain size decreases, the higher density of the three-phase- boundary (TPB) length l(TPB) leads to an increase in the overall current density and a reduction in the time to reach the steady-state current density t(st), thus causing high power density. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3097190] All rights reserved.