Three different copper zeolite systems (Cu-ZSM5, Cu-UHSY, and Cu-mordenite) have been prepared with different copper loadings from low to over exchange levels. The surface chemistry of three systems has been studied using IR spectra of adsorbed NO. Most of the copper in Cu-ZSM5 was in the form of Cu(I). Cu(II) species were predominant in Cu-mordenite. For Cu-UHSY the number of Cu(I) species increased as the level of copper loading increased. The catalytic activity for NO decomposition appeared to depend on a Cu(I)- redox cycle which resulted in the formation of an ON-Cu2+-NO2- complex, previously postulated as the critical intermediate in the reaction. Both oxidation and regeneration of the Cu(I) sites in Cu-ZSM5 occurred readily. In Cu-UHSY oxidation was low, suggesting that Cu(I) species were more stable. Cu(II) was the prominent copper species in Cu-mordenite, and regeneration of the Cu(I) sites was difficult. Stability, oxidation, and regeneration of Cu(I) sites play a major role in NO decomposition. In over-exchanged Cu-ZSM5, the Cu(I) sites were more stable, and the rate of oxidation was high. Furthermore a large number of reduced sites were readily regenerated from the Cu(II), thus making it the most effective catalyst for the decomposition of NO.