Fe was introduced in MFI molecular sieves by four means : direct synthesis, postsynthetic modification of boralite, ion exchange with HZSM-5 in solid or liquid state, and by impregnation. Fe species were characterized by TPR and EPR (dispersed Fe3+). Additional information was obtained from the FTIR spectra of adsorbed acetonitrile-d(3) and from TPD of ammonia, both of which characterized the acidity of the samples employed. The catalytic activity was examined in oxidative dehydrogenation of ethane, in aromatization of ethane and ethene, in CO oxidation by O-18(2) and in O-18(2) + O-16(2) reactions. The results obtained were compared with those over Ga metallosilicates : it was found that Fe in the zeolite framework appeared only in directly synthesized samples. These were much more active in oxidative dehydrogenation of ethane than HGaZSM-5, but much less active in the aromatization reactions. The oxidation of CO with gaseous oxygen began < 100 degrees C, contrary to Ga samples over which the onset of reaction needed a much higher temperature. Significant activation of oxygen molecules, leading to the equilibration of gaseous oxygen molecules as well as to the exchange with lattice oxygens, was observed at 400 degrees C over the most active ferrisilicate.