The oxygen reduction mechanism of dense mixed-conducting La1-xSrxCo0.2Fe0.8O3-delta (LSCF) electrodes with various Sr contents was investigated by employing ac-impedance spectroscopy and potentiostatic current transient (PCT) technique. From the analyses of the ac-impedance spectra and the cathodic PCTs measured on the electrodes as functions of electrode thickness l and oxygen partial pressure p(O2), it is confirmed that the overall oxygen reduction reaction mainly proceeds by oxygen vacancy diffusion through the electrode coupled with the charge-transfer reaction on the electrode surface. Furthermore, as Sr content increases, the decrease in the diffusion resistance R-d due to higher diffusivity of oxygen vacancy D-VO is more predominant than that in the charge-transfer resistance R-ct due to the larger electrochemically active area A(ea). The smaller values of R-d and R-ct with increasing Sr content lead to the higher initial current I-ini, higher steady-state current I-st, and shortened time to reach steady-state current t(st) in the cathodic PCTs. From the analyses of the impedance spectra combined with the cathodic PCTs measured on the electrodes cathodically polarized for various times, it is suggested that the decomposition of SrO on the electrode surface by the cathodic polarization enhances A(ea), resulting in the kinetic facilitation of the charge-transfer reaction at the electrode/gas interface. (c) 2007 The Electrochemical Society.