The interaction of nitrogen monoxide with various types of iron-containing pentasilic zeolites has been investigated by EPR spectroscopy. The systems investigated were iron-silicalite, ion-exchanged ZSM-5 and a bare silicalite impregnated with iron ions at the external surface only. NO forms paramagnetic adducts on all these systems essentially reacting with Fe(II) species. Three distinct types of nytrosyl adducts; have been identified all lacking the hyperfine structure. Two of them are in doublet (S = 1/2) state while the third one, observed in ZSM-5 samples only and already reported in the literature, is a quadruplet (S = 3/2). While all activated samples exhibit EPR spectra (due to Fe(III) ions) very similar one to each other, their reactivity towards NO is different in each case. This allows some advance in understanding the state of the activated samples which, due to the high importance of iron-containing pentasilic zeolites in heterogeneous catalysis, is the object of a active debate in the literature.