For a range of Cu-ZSM-5 catalysts with different Cu-exchange levels on the two kinds of ZSM-5 with different Si/Al ratios, temperature programmed reduction using CO (CO-TPR) followed by H₂(H₂-TPR), and temperature programmed desorption of oxygen (O₂-TPD) were conducted using an online mass spectrometer to characterize and quantify the copper species on the catalysts in the calcined state. Copper species on the ZSM-5 were quantitatively characterized as Cu²⁺, (Cu-O-Cu)²⁺ and CuO after calcination in oxygen environment. The N₂ formation activities of the catalysts in the decomposition of NO were well correlated with the quantified catalytic amounts of the Cu²⁺ ions involved in the Cu-dimers, (Cu-O-Cu)²⁺. The mol fraction of the Cu ions present as the Cu-dimers increased at the sacrifice of the isolated Cu²⁺ with increasing Cu ion exchange level, suggesting that the species could be formed between the two Cu²⁺ in close proximity. Oxygen that could be thermally desorbed from the oxidized catalysts in the O₂-TPD was responsible for the reduction of the Cu-dimers. It was concluded that the decomposition of NO over Cu-ZSM-5 catalyst proceeded by the redox of (Cu-O-Cu)²⁺, as active centers. With the temperature programmed surface reaction using N₂O or NO over an oxidized catalyst sample as well as the O₂-TPD, it was possible to estimate the change of the oxidation state of the Cu ions engaged in the Cu-dimers.