In summary, the results obtained allow us to conclude that the two BaFeO3- oxides with The influence of the initial vacancy concentration and iron oxidation state on the catalytic response of the Ba-Fe-O system in the CO oxidation process has been studied by comparing the BaFeO2.78 and BaFeO2.90 samples with similar textural characteristics. A direct comparison of the light-off temperatures shows that BaFeO2.90 shows higher CO conversion, supporting the Fe 4+/ Fe 3+ ratio as the predominant factor of the catalytic activity over the oxygen vacancies concentration. The obtained results show up the Ba-Fe-O system, in particular the BaFeO2.90 6H phase, as an outstanding noble-metal-and lanthanide-free catalyst, for CO conversion.