In situ XAFS spectroscopy was applied in order to determine the catalytically active sites in Fe/ZSM5, prepared by FeCl3 sublimation. The number of (inactive) spectators in this catalyst was minimized by using a specially dedicated calcination procedure. The catalytic activity of Fe/ZSM5 during the collection of the XAFS data was monitored via chemiluminescence analysis of the gas outlet. Binuclear Fe complexes with a Fe-O-Fe core were found to be catalytically active species in this material. During heat treatment in He to 350 degreesC the Fe complexes undergo auto-reduction, ascribed to the removal of oxygen from Fe-O-Fe bridges (closest Fe-O-coordination sphere) and the formation of Fe-rectangle-Fe vacancies. Treatment with isobutane results in a further slight average reduction of iron, accompanied by an additional removal of oxygen from the Fe-O-Fe bridges. Fe in the binuclear complexes is readily oxidized when NO, NO + O-2, or a typical HC-SCR mixture (NO, i-C4H10, O-2) is fed to the Fe/ZSM5 catalyst. Reoxidation is accompanied by filling of the Fe-rectangle-Fe vacancies. Under HC-SCR working conditions the average oxidation state of iron is 3+. (C) 2003 Elsevier Inc. All rights reserved.