The interaction of N2O with MFI zeolites was investigated by a multipulse transient response method at 250 degrees C with subsequent temperature-programmed desorption. It was shown that the active sites for the formation of a surface oxygen species can be determined quantitatively using this method under the conditions of catalytic applications. Iron content and several pretreatment procedures influenced the formation of surface oxygen and molecular N2O sorption. For example, fresh zeolites with high and low iron content formed significantly more surface oxygen than zeolites that had been pretreated in N2O. The calcination at higher temperatures had an effect only on the activity of the zeolites with high iron content, which was not as pronounced as has been reported in the literature. The adsorption of molecular N2O as an elementary step of N2O activation in iron-containing zeolites cannot be neglected, because it was found to be thermally quite stable. All results indicate that in these zeolites not only one, but several oxygen species were formed that differ in thermal stability and thus in reactivity. (c) 2005 Elsevier Inc. All rights reserved.