The oxygen reduction reaction on mixed conducting (La0.85Sr0.15)(0.9)MnO3 electrodes with various porosities was investigated by analysis of the ac-impedance spectra. To attain a mixed electronic/ionic conducting state of (La0.85Sr0.15)(0.9)MnO3 with high oxygen vacancy concentration, the electrode specimen was purposely subjected to cathodic polarisation. The ac-impedance spectrum clearly showed a straight line inclined at a constant angle of 45 degrees to the real axis in the high-frequency range, followed by an arc in the low-frequency range, i.e. it exhibited the Gerischer behaviour. This strongly indicates that oxygen reduction on the mixed conducting electrode involves diffusion of oxygen vacancy through the electrode coupled with the electron exchange reaction between oxygen vacancies and gaseous oxygen (charge transfer reaction) at the electrode/gas interface. It was further recognised that the two-dimensional electrochemical active region for oxygen reduction extends from the origin of the three-phase boundaries (TPBs) among electrode, electrolyte and gas into the electrode/gas interface segments, which is on average approximately 0.7 to 1.1 mu m in length below the electrode porosity 0.12. Based from the fact that the ac-impedance spectrum deviated more significantly from the Gerischer behaviour with increasing electrode porosity above 0.22, it is proposed that due to the increased length of TPBs, the rate of the overall oxygen reduction on the highly porous electrode is mainly determined by the charge transfer reaction at the TPBs, and the subsequent diffusion of oxygen vacancy occurs facilely through the electrode.