Preparation of steam-activated Fe-silicalite containing mainly isolated iron species in extraframework positions was essential for deriving structure-activity relationships in various N2O conversion reactions over iron zeolite catalysts. Characterization by UV-Vis/DRS revealed that any significant clustering of iron did not occur in this sample. Other steam-activated FeMFI zeolites, with different framework compositions or treated at higher temperatures, showed various degrees of clustering. The activity of the cluster-free Fe-silicalite was significantly higher in N2O reduction with C3H8 and CO. However, some level of association of iron species leads to higher activities in direct N2O decomposition. Due to the intrinsic reaction mechanism, this result demonstrates the sensitivity of reactions for the form of the iron species in Fe-zeolites, rather than the existence of a unique active site. (C) 2003 Elsevier Inc. All rights reserved.