The surface active Fe-sites in HZSM-5 with low content of iron (< 1000 ppm) activated by steaming and high temperature (up to 1323 K) calcination in inert lead to the formation of surface oxygen (O)(ad) species from N2O and were characterized quantitatively by transient response method. Only a part of (O)(ad) deposited on zeolite by decomposing N2O was active in CO oxidation at 523 K. A binuclear Fe-center is suggested as an active center, featuring a "diamond core" structure, similar to that of the monooxygenase enzyme. The active O-atoms were assigned to the paired terminal oxygen atoms each bound to one Fe-site in the binuclear [Fe2O2H](+)-cluster. Zeolite pre-saturated by water vapor at 473-523 K generates (O)(ad) species from N2O completely inactive in the CO oxidation. The total amount of the oxygen, (O)(ad), deposited on the pre-saturated by water zeolite corresponds to a half of stoichiometric amount of the surface Fe-atoms and suggests that water blocks a half of the binuclear [Fe2O2H](+)-center, the remaining acting as a single Fe-site. (C) 2004 Elsevier B.V. All rights reserved.