A microscopic study of the deactivation of Cu-ZSM-5 was carried out. The activity of Cu-ZSM-5 for NOx reduction with hydrocarbons decreased when heated above 600 degrees C in a simulated net oxidixing exhaust gas. Al-27 and Si-29 nuclear magnetic resonance results revealed that a dealumination of the zeolite occurs in the deactivated Cu-ZSM-5. Electron spin reonance (ESR) results indicated that aggregation of copper ions did not occur and copper remained as isolated ions in Cu-ZSM-5 even after the deactivation. The ESR spectra of dehydrated Cu-ZSM-5 suggested the migration of copper ions in the zeolite after the deactivation. To investigate the interactions between copper ions and NO/C3H6, the ESR spectra were measured after exposure to nitric oxide and propene. The ESR results indicated that the oxidation state and coordination structure of the copper ions were changed by the adsorption of nitric oxide or propene in active Cu-ZSM-5. On the other hand, no such reaction was observed in the deactivated Cu-ZSM-5. In the deactivated Cu-ZSM-5, copper ions remain atomically dispersed but cannot interact with nitric oxide or propene. In conclusion, the deactivation of Cu-ZSM-5 is suggested to occur through the migration of copper ions to sites where gas molecules like nitric oxide and propene cannot reach them. This migration is triggered by the dealumination of the zeolite.