The catalytic properties of Fe3+ in the framework sites of MFI-type ferrisilicate were studied by comparing with iron-oxide-impregnated silicalite (FeO(x)/Sil) for the oxidation of CO with 02. On FeO(x)/Sil the reaction proceeds via a redox mechanism where CO is oxidized with the lattice oxygen of the catalyst (iron oxide), while on ferrisilicate, the CO oxidation occurs by the reaction of CO with the oxygen from the gas phase. These mechanisms were confirmed by kinetic and isotopic tracer results. The CO oxidation with O-18(2) on ferrisilicate revealed significant incorporation of lattice oxygen into product CO2 produced. The CO oxidation mechanism on ferrisilicate was explained as follows : CO2 is produced by reaction between CO and gas phase oxygen; heterophase exchange between lattice oxygen and CO2 occurs at a significant rate; furthermore, oxygen isotope equilibration in CO2 occurs simultaneously. On FeO(x)/Sil, the amount of lattice oxygen incorporated into CO2 during CO oxidation with O-18(2) was very small. The CO oxidation mechanism on FeO(x)/Sil was explained as follows : lattice oxygen in the surface layer of Fe2O3 crystallite in FeO(x)/Sil oxidizes CO into CO2, and the reduced surface site is rapidly reoxidized with gas phase oxygen. As the reaction proceeds, the surface lattice oxygen is almost replaced by gas phase oxygen because the scrambling of the lattice oxygen between the surface layer and the bulk of Fe2O3 occurs much more slowly than CO oxidation. 1994 Academic Press, Inc.