This work was attempted in order to provide a new and well-established approach for the quantification of Fe(2+) sites in zeolites. The experimental procedure was based on carbon monoxide and nitrogen monoxide sorption in Fe-exchanged ZSM-5 zeolites with different pore hierarchies, which was followed by IR spectroscopy. The values of the absorption coefficients of the respective monocarbonyl Fe(2+)(CO) and mononitrosyl Fe(2+)(NO) IR bands were obtained. In turn, the concentrations of Fe(ii) moieties of various kinds were calculated and discussed with regard to the composition (Si/Al and Fe/Al ratios) of the zeolites studied.