The formation of paramagnetic complexes between Fe2+ ions and NO molecules with total spin S = 3/2 has been observed in FeZSM-5 zeolites. These complexes exhibit a typical ESR signal with a component at g = 4.0 and are spectroscopically similar to those observed in iron-containing enzymes. Such spin states of Fe ions have not been reported for heterogeneous catalysts. The application of NO as a spin probe for investigation of the state of low-valence Fe ions in zeolite matrices is discussed.