Exposure to oxygen of Fe/MFI catalysts, prepared by sublimation, results in an adsorbed oxygen species capable of reacting with nitrogen oxide or carbon monoxide at room temperature. The IR bands in the perturbed T-O-T vibration region indicate oxidation of Fe2+ at room temperature. The FTIR data also show that more nitro groups and nitrate ions are formed from NO + O-2, if Fe/MFI is exposed to oxygen first, than if nitrogen oxide is dosed first. Formation of these NOy groups is faster on pre-reduced than on pre-oxidized catalysts, which suggests that diatomic oxygen is adsorbed on sites which, in the pre-oxidized catalysts, are occupied by oxide ions. Over pre-reduced catalysts, carbon monoxide oxidation with O-18(2) yields significant amounts of (COO)-O-18-O-16 and a small amount of (CO2)-O-18, even at room temperature. It is uncertain whether the adsorbed diatomic oxygen is present as a superoxide ion, as previously detected by ESR on Fe/MFI, Co/MFI and V/MFI at 77 K, or as a peroxo groups bridging over two iron ions, as identified in DFT analysis. Formation of nitrate ions is visualized by interaction of nitrogen oxide with a superoxide ion, possibly via a peroxynitrite ion.