Electron spin resonance spectroscopy has been used to study the adsorption of NO over copper ion exchanged zeolites. These studies are especially useful if different electron spin resonance frequencies (X-band, Q-band) are applied. In the present study, we have synthesized ZSM-5 (MFI, structure code of the International Zeolite Association (IZA)), MCM-22 (MWW) and MCM-58 (IFR) zeolites with various n(Si)/n(Al) ratios and ion exchanged with copper acetate after calcination. The materials were characterized by X-ray powder diffraction. The various zeolites were then subjected to high vacuum conditions. The Cu(I)-NO complexes were formed over these pretreated copper exchanged zeolites by NO adsorption at 77 K. The electron spin resonance spectra revealed the existence of two types of Cu(I)-NO complexes with slightly different g tensor and copper hyperfine parameters. The influence of n(Si)/n(Al) ratio over the formation of these Cu(I)-NO complexes and the temperature dependence of the intensity of their electron spin resonance signal were studied over all zeolites under investigation. (C) 2004 Elsevier B.V. All rights reserved.