Ammonia oxidation to nitrogen and water over CuY zeolites has been studied in the temperature range from 160 to 185 ^oC. The reaction rate was first order in NH3 and zero order in O2. The rate was proportional to the degree of Cu^2^+ ion exchange up to a level of about one ion per large cavity. An activation energy of 36.7 +/- 2.6 kcal/mol was observed for the three exchange levels which were studied (3, 14, 44%). For the 14%-exchanged sample a turnover number of 0.169 NH3 molecules per minute per Cu^2^+ ion was observed at 175 ^oC and an ammonia pressure of 300 Torr. Infrared and electron paramagnetic resonance studies indicate that a [Cu(II) (NH3)4]^2^+ complex was formed in the large cavities of the zeolite. The slow step in the reaction mechanism was the reduction of Cu(II) to Cu(I) by the NH3. The reoxidation of the Cu(I) complex to a Cu(II) complex was very rapid under the reacting conditions. A detailed mechanism is proposed which is consistent with the experimental data. These results show that Cu^2^+ ions can be extracted from hidden positions within the zeolite framework to form complexes within the large cavities of the zeolite where catalytic reactions can freely occur.