Reactions of copper oxide cluster cations, CunOm+ (n = 3-7; m <= 5), with ammonia, NH3, are studied at near thermal energies using a guided ion beam tandem mass spectrometer. The single-collision reactions of specific clusters such as Cu4O2+, Cu5O3+, Cu6O3+, Cu7O3+, and Cu7O4+ give rise to the release of H2O after NH3 adsorption efficiently and result in the formation of CunOm-1NH+. These CunOm+ clusters commonly have Cu average oxidation numbers of 1.0-1.4. On the other hand, the formation of CunOm-1H2+, i.e., the release of HNO, is dominantly observed for Cu7O5+ with a higher Cu oxidation number. Density functional theory calculations are performed for the reaction Cu5O3+ + NH3 -> Cu5O2NH+ + H2O as a typical example of H2O release. The calculations show that this reaction occurs almost thermoneutrally, consistent with the experimental observation. Further, our experimental studies indicate that the multiple-collision reactions of Cu5O3+ and Cu7O4+ with NH3 lead to the production of Cu-5(+) and Cu7O+, respectively. This suggests that the desirable NH3 oxidation to N-2 and H2O proceeds on these clusters.