The adsorption of the biologically important signaling molecule nitric oxide (NO) on two metal–organic frameworks (MOFs), Cu3btc2 and Cu3(NHEtbtc)2, has been studied by multinuclear solid-state NMR. N-Diazenium diolate (NONOate) formation as primary mode of NO adsorption in the latter MOF is concluded from 15N, 1H, and 13C NMR spectra together with analysis of the 1H spin–lattice relaxation time (T1). Furthermore, NO adsorption at the open metal site of Cu3btc2 as well as Cu3(NHEtbtc)2 is also evident, indicating both physisorption and chemisorption of nitric oxide.