The structure of the iron species in mildly calcined over-exchanged Fe/ZSM5, prepared by CVD of FeCl3, was studied during heat treatments in He or O-2/He (50:50) by coupling in situ Fe K edge HR-XANES and EXAFS. The majority of iron appears to be present as Fe-binuclear complexes. EXAFS shows that the closest Fe-O shell in the complexes can be described with a [HO-Fe-O-Fe-OH](2+) core. Heating to a moderate temperature (up to similar to150degreesC) results in the desorption of water from the Fe-coordination sphere, in He as well as in a O-2/He (50:50) mixture. The composition of the gas phase strongly influences the changes occurring to the binuclear complexes in the 150-350degreesC temperature range. Above 250degreesC in He a significant fraction of iron undergoes autoreduction. This is accompanied by the removal of approximately one oxygen atom from the closest Fe-O shell, ascribable to the Fe-O-Fe bridging oxygen atom. The presence of oxygen in the gas phase (PO2 = 0.5 bar), on the contrary, suppresses the auto-reduction capability of the binuclear complexes by inhibiting the removal of the Fe-O-Fe bridging oxygen. (C) 2003 Elsevier Science (USA). All rights reserved.