This work presents the study of the O-2-Reduction Reaction (ORR) by electrochemical impedance spectroscopy of La0.6Sr0.4Co0.8Fe0.2O3-delta and La0.6Sr0.4Co0.2Fe0.8O3-delta cathodes as a function of temperature and pO(2). The combination of the impedance data, modeled with a Transmission Line Model, with the microstructural data obtained by FIB-SEM tomography, allowed to obtain and compare the chemical diffusion coefficients (Dchem), O-2 equilibrium molar exchange rates (R-0) and the oxygen surface exchange rates (k(chem)) for both compounds. The obtained values were, at 700 degrees C in air, Dchem = 5.4.10-7 cm2.s-1 and k(chem) = 1.4.10(-6) cm.s(-1) for La0.6Sr0.4Co0.2Fe0.8O3-delta, while Dchem = 2.6.10-6 cm(2).s(-1) and k(chem) = 3.1.10(-)6 cm.s(-1) were obtained for La0.6Sr0.4Co0.8Fe0.2O3-delta. The detailed analysis of these parameters as a function of pO(2) (10(-4) < pO(2) = 1) and temperature (500 degrees C = T = 700 degrees C) by means of the Adler-Lane-Steele model, adapted to a finite length porous electrode, allowed identifying the O-ion diffusion and surface exchange as processes co-limiting the ORR. From this analysis, a predominantly surface limited ORR was found for La0.6Sr0.4Co0.2Fe0.8O3-delta, changing to a more bulk limited ORR for La0.6Sr0.4Co0.8Fe0.2O3-delta d which has higher oxygen-vacancy concentration. (C) 2019 The Electrochemical Society.