Decomposition of N2O has been studied in the temperature range 293 < T < 673 K over Fe-ZSM-5 with 0.3 wt% extra-framework Fe after prereduction in H-2/Ar or Ar flow at 473 < T < 973 K. Two different processes of reoxidation of reduced Fe2+ species by N2O were identified: (i) a fast one starting at approximate to 500 K involving deposition of atomic oxygen on isolated Fe2+ species, leading to the formation of Fe3+O center dot- sites detected by EPR spectroscopy (site 1), and (ii) a slow one related mainly to the reoxidation of small FexOy clusters (site 2), the formation of which cannot be avoided in H2/Ar flow. The number of sites I increases with rising prereduction temperature, whereas sites 2 are independent of T-red.. O center dot-formation could not be detected on reoxidation of Fe species by O-2. For the first time, a direct relation between Fe2+ reoxidation by N2O and formation of O center dot- could be detected by EPR spectroscopy, suggesting that the so-called "alpha-oxygen" first described by Panov et al. is an O center dot- radical anion. Based on in situ XANES experiments, this makes it highly unlikely that the previously discussed ferryl moiety (Fe4+=O) is the source of the highly active oxygen. (C) 2007 Elsevier Inc. All rights reserved.