Accessible iron in oxidized Fe-ZSM-5 and Fe-BEA zeolites is mainly in the form of Fe3+ species that do not interact with CO. Some exchanged cations are stabilized as Fe2+ species and form carbonyls detected in the 2205-2190 cm(-1) region. Evacuation of the samples at 673 K provokes autoreduction of part of Fe3+ and creation of more Fe2+ sites detectable by CO. Reduction of the samples with CO at 673 K leads to a significant increase of the number of Fe2+ sites and creation of new reduced iron sites (supposed to be Fe+ cations). With CO the latter forms tri- and tetra-carbonyl complexes. In order to clarify the structure of these species, adsorption of CO isotopic mixtures is studied. It is demonstrated that the use of (CO)-C-12-O-16-(CO)-C-13-O-18 mixture provided a better resolution of the carbonyl bands as compared to the conventional (CO)-C-12-O-16-(CO)-C-13-O-16 mixture. (C) 2010 Elsevier B.V. All rights reserved.