The interaction of O-2 with Fe supported on zeolite MFI has been investigated by the in situ W Raman technique. Previously, the formation of adsorbed superoxide ions, O-2(-), On the Fe/MFI prepared by sublimation of FeCl3 vapor onto HMFI, was identified by ESR at 77 K. In situ Raman data indicate that adsorbed peroxide ions, O-2(2-), are formed on the same catalysts with Fe/Al = 1 even at 300 K. A band at 730 cm(-1), which was found to be sensitive to the partial pressure of O-2, is tentatively assigned to the stretching vibration of peroxide ions. With the,O-18(2) isotope, a red shift of 32 cm(-1) was observed for the 730 cm(-1) band. Raman features of other catalyst samples show that the binuclear iron sites in the Fe/MFI are essential for the formation of peroxide ions. As proposed before, the same bridging sites between two Fe ions, that are occupied by O2- in the calcined catalyst, are also able to adsorb di-oxygen complexes.