The spin state of iron ions in Fe-ZSM-5 zeolites can be purposefully varied by adsorption of gaseous probe molecules. The resulting Fe complexes with half-integer spin (S = 1/2, S = (3)/(2) and S = (5)/(2)) can be reliably identified by electron paramagnetic resonance (EPR). A good correlation has been found between the concentration of surface sites active in low-temperature nitrous oxide decomposition and the concentration of low-spin (S = 1/2) nitrosyl complexes of Fe formed after adsorption of NO molecules. Based on the analysis of the formation of such complexes under varying conditions, we conclude that these active sites contain a binuclear iron complex with S = 0 and three adsorbed NO molecules. An approach to investigate various Fe-containing sites in oxidation catalysts is discussed. (c) 2005 Elsevier B.V. All rights reserved.