The redox features of copper ion exchanged in ZSM-5 were investigated in both evacuation-oxidation-re-evacuation and evacuation-hydration-re-evacuation processes. The XANES and ESR spectra clearly indicated the conversion of Cu(II) ion exchanged in ZSM-5 into Cu(I) with increasing evacuation temperatures. The ability of Cu(I) formed to undergo a complete oxidation with O-2 at 673 K was certified and succeeding reduction process by heat treatment in vacuo was examined. Adsorbed amounts of CO and N-2 at 298 K are scarcely changed for the samples evacuated at 873 K before and after O-2 treatment. The extended X-ray absorption fine structure (EXAFS) spectrum exhibited a band centered at around 2.2 Angstrom through the exposure of the CuZSM-5 sample evacuated at 873 K to water vapor. This band was assigned to the backscattering from the first nearest copper metal, indicating the formation of small metal particles. The production of the divalent copper ion was also evidenced by the ESR data. These results suggest that the Cu(I) species formed in the evacuation process are transformed into the Cu(II) species and metal particles; the disproportionation reaction occurs, 2Cu(I) --> Cu(II) + Cu(O). As the results, it is concluded that the redox process between Cu(II) and Cu(I) is completely reproducible in the evacuation-oxidation-re-evacuation process and this is the key process in the catalytic decomposition of nitric oxides on CuZSM-5: the high stability of Cu(I) in ZSM-5. In addition, one of the causes of the poisoning by water adsorption in catalytic decomposition of NO, may be due to the release of Cu(I) in ZSM-5 through the disproportionation reaction. (C) 2002 Elsevier Science B.V. All rights reserved.