TPD, TPR, UV-visible spectroscopy, and high-resolution electron microscopy (HREM) have been used to characterize the state and reactivity of alumina-supported copper-based catalysts for the oxidation of ammonia to nitrogen. The results of HREM and UV spectra show that a CuAl2O4-like phase is more active than a CuO phase for the ammonia oxidation reaction. Both surface oxygen and copper lattice oxygen can react with NH3 to produce N-2 but surface oxygen is much more active than lattice oxygen at low temperature. For copper zeolite catalysts [Cu-O-Cu](2+)-like species or small copper oxygen aggregates are the likely forms of the catalytically active centers at low temperature. The activity of CuY was increased by treating the sample with NaOH. This treatment presumably increases the amount of low temperature active centers.