The aim of this work is to compare the reactivity and the transformation of the Fe-Co alloy on the one side and the Co-containing magnetite on the other side in Fe-Co/Co-containing magnetite catalysts after CO/H-2 and CO2/H-2 reactions. Different Co-to-Fe ratios are studied. The catalysts undergo different modifications owing to their initial composition, but especially to the gas mixtures. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) show that iron and cobalt carbides are formed in both cases: the iron carbides are formed first and are located on the particle surface while the cobalt carbides are formed in a second step and localized in the center. The spinel lattice parameter keeps constant values in CO/H-2, while it decreases in CO2/H-2. Furthermore the CO/H-2 reaction occurs without a preliminary reduction unlike under CO2/H-2 for which it is necessary. We propose that the carbides came preferentially from the Fe-Co alloy under CO/H-2 and from the spinel under CO2/H-2. CO2 formation during CO/H-2 reactions could be related to the change of Fe-Co alloy and of the spinel.