A series of La0.8Sr0.2CoxFe1-xO3-delta (LSCF) perovskite oxides, including La0.8Sr0.2CoO3-delta (LSC) and La0.8Sr0.2FeO3-delta (LSF), were synthesized via a combustion method and, mixed with carbon black at a mass ratio 3:1, were studied towards their oxygen reduction reaction (ORR) activity in 0.1 M KOH solution at room temperature, using the thin-film rotating disk electrode (RDE) technique. The highest and lowest ORR (specific and mass) activities among the tested electrocatalysts were exhibited by LSC and LSF, respectively. Increasing Co content and at the same time decreasing Fe content in the LSCF composition, resulted in ORR specific activity enhancement as well as in an increase in the number of transferred electrons, from ca. 2 (for LSF) to ca. 4 (for LSC), implying a change in the ORR mechanism. The observed change in ORR activity of the perovskite/C electrocatalysts upon gradual substitution of iron by cobalt was associated with induced changes in the surface B-sites electronic structure and in surface oxygen vacancy formation.