The reactivity in oxygen of several mixed valency ferrite spinels, namely Fe3O4, FeCr2O4, Fe3-xTixO4 and Fe3-xMoxO4 was investigated by derivative thermogravimetry (DTG) analysis as a function of the conditions of preparation and grinding. For all these compounds, low temperature preparation or prolonged grinding time enables small particles (< 0.3 mu m) of gamma-spinel phase with a high content of defects to be produced. In contrast, for unground or high temperature annealed samples having a larger crystallite size (> 0.5 mu m), it was observed that oxidation was starting at higher temperature and that the defect spinel phases could not be retained during oxidation. This behaviour was attributed to the presence of stresses induced by the lattice parameter gradient and promoting the formation of nuclei of the alpha-rhombohedral phase from the superficial gamma-defect phase.