The selective catalytic reduction of nitric oxide with ammonia in the presence of oxygen has been studied on Cu2+ ion-exchanged zeolite catalysts. In the case of Cu2+-exchanged ZSM-5, the reaction proceeded selectively at lower temperatures, where nitric oxide and ammonia reacted with one-to-one stoichiometry to produce nitrogen. At higher temperatures, an oxidation of ammonia with oxygen occurred concomitantly to reduce the conversion of nitric oxide. The formation rate of nitrogen was first-order with respect to the partial pressure of nitric oxide, nearly half-order to that of oxygen, and zero-order to that of ammonia. The apparent activation energies were almost the same for all Cu-ZSM-5 examined having different Cu2+-exchange levels and Si/Al atomic ratios. The ideality of Cu2+-exchange ascertained from the stoichiometry of ion-exchange for the exchange levels less than 200% suggested the atomic dispersion of copper species. The specific activity per Cu2+ ion increased with increasing ion-exchange level from 29 to 195% and also with decreasing the Si/Al atomic ratio from 71 to 23. The specific activities of various Cu-ZSM-5 were found to depend on the concentration of the Cu2+ ions; i. e., the higher the copper concentration, the higher the specific activity. The active copper species were proposed to be paired Cu2+ species in view of the relation between the specific activity and the copper concentration.