The chemisorption state of NO adsorbed on Cu/ZSM-5 was investigated by the TPD method in an UHV system and was discussed to be correlated with the NO decomposition mechanism. The three distinct states of NO adsorbed on Cu/ZSM-5 (T-ad = 175 K) are indicated by the appearance of the peaks at 210 (alpha), 330 K (beta) and 480 K (gamma) with increasing exposure. It is found that the beta peak is attributable to NO adsorbed on Cu+ ions, while the cu peak is assigned to NO physically adsorbed on support. At high exposure pressure, a new peak is developed and N-2 and N2O are produced during TPD, suggesting that a dinitrosyl species as an intermediate species for NO decomposition is formed. (C) 1997 Elsevier Science B.V.