The present paper shows a direct comparison of IR spectra of adsorbed NO on two catalyst systems (Fe-silicalite and Fe-ZSM-5), recorded in a flow-through cell (in operando, where NO is carried by an excess of inert gas) and static cell (in situ, where a given pure NO equilibrium pressure is dosed). A progressive NO poly-adsorption is observed in the static cell upon increasing the NO equilibrium pressure (from Fe2+(NO)2 to Fe2+(NO)3), while predominantly Fe2+NO adducts are observed in the flow-through cell. By comparing literature spectra, it is shown that these spectral differences are intrinsically inherent to the two different experimental approaches. The two experimental set-ups are able to observe preferentially only a part of the total Fe species present on Fe-zeolites. Water contamination experiments employing different experimental conditions (order of dosage, co-dosage, different NO/H2O ratios) did not reproduce in the static environment the IR spectra collected with the dynamic set-up. The spectral differences could have a thermodynamic origin and be related to different adsorption enthalpies of mono- and di-nitrosyl complexes of the two Fe2+ families and to the different NO partial pressure adopted in the two experimental configurations. These considerations have important consequences for the conclusions reached by different research groups on the structure of active Fe sites.