A kinetic study of the oxidation of Mo3+ ions has been performed on two submicrometer Mo and Mo-Co ferrites of spinel structure with a nearly identical molybdenum content. We show that for the Co-free sample, oxidation proceeds by cationic diffusion in a cation-deficient spinel with a variable chemical diffusion coefficient. For the Co sample, which showed a pronounced stability towards oxygen, the oxidation process was divided into three steps. In step I, oxidation declines with time in an initial period (degree of oxidation alpha < 0.1). In step II (0.1 < alpha < 0.25), oxidation is accelerated and in step III (alpha > 0.25) oxidation is best described by the diffusion-controlled Jander equation, Comparing the vibrational spectra of these two spinels after total oxidation for various thermal treatments, we conclude that the coordination around Mo6+ ions changes from tetrahedral to octahedral with Co substitution and that the location of cobalt and molybdenum ions in octahedral sites changes with the annealing process.