The oxygen reduction mechanism was investigated at the porous LSM-patterned YSZ composite electrode by employing the ac-impedance spectroscopy and the potentiostatic current transient (PCT) technique. For this purpose, the dense YSZ pellet was patterned by a laser beam, and was then coated with the LSM slurry. The length of three-phase boundaries (TPBs) per unit area l(TPB) was effectively controlled by varying the width of the YSZ column. From analyses of the ac-impedance spectra and the cathodic PCTs obtained from the electrodes based upon the modified transmission line model (TLM), it was first experimentally confirmed that the effective migration length l(m) decreased with increasing l(TPB) under the mixed migration and charge-transfer control. Secondly, as the value Of I-TPB increases. the charge-transfer resistance R-ct is decreased to a more extent but the ion migration resistance R-i is reduced to a less extent. Finally, from a comparison of the cathodic PCTs measured on the porous LSM-YSZ composite electrode to those measured on the porous LSM-patterned YSZ composite electrode, the oxygen reduction kinetics at that porous composite electrode was discussed in terms of the steady-state current density i(st) and the time to reach the steady-state current density t(st). (c) 2008 Elsevier Ltd. All rights reserved.