The CuO/Al2O3 system is active for ammonia oxidation to nitrogen and water. The principal by-products are nitrous oxide and nitric oxide. Nitrous oxide levels increase with the addition of various metal oxides to the basic copper oxide/alumina system. Addition of sulfur dioxide to the reaction stream sharply reduces the level of ammonia conversion, but has a beneficial effect on selectivity to nitrogen. Added water vapour has a lesser effect on activity but is equally beneficial in terms of selectivity to nitrogen. The CuO/Al2O3 is also active for the selective catalytic reduction of nitric oxide by ammonia, but this reaction is not effected by sulfur dioxide addition. A mechanism for ammonia oxidation to nitrogen is proposed wherein part of the ammonia fed to the catalyst is converted into nitric oxide. A pool of monoatomic surface nitrogen species of varying oxidation states is established. N-2 or N2O are formed depending upon the average oxidation state of this pool. An abundance of labile lattice oxygen species on the catalyst surface leads to overoxidation and to N2O formation. On the other hand, reduced lability of surface lattice oxygen species favours a lower average oxidation state for the monoatomic surface nitrogen pool and leads to N-2 formation. (C)2000 Elsevier Science B.V. All rights reserved.