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The electrochemical activity towards the oxygen reduction reaction (ORR) of carbon supported Ba0.5Sr0.5CoxFe1-xO3-δelectrodes particles is studied for the first time as a function of chemical composition (1<x<0). Highly crystalline Ba0.5Sr0.5CoxFe1-xO3-δ particles with the full compositional range were synthesized by an ionic-liquid route, leading to a high degree of phase purity. The structure and composition of the materials were investigated by quantitative XRD and EDX. Electrochemical studies in oxygen-saturated alkaline solutions show that the overall reactivity of the catalyst supported on mesoporous carbon increases as the Co content increases above 50%. DFT calculations, based on the structural parameter obtained from XRD refinements, show that the oxygen-vacancy formation energy decreases as the Co content increases. Oxygen vacancies lead to a higher electron density at the Co atoms, which are identified as the key ORR active sites.