The reaction mechanism for the selective catalytic oxidation of ammonia to give nitrogen on CuO/Al2O3, Pt/Al2O3 and Pt/CuO/Al2O3 was investigated using transient experiments. Temperature-programmed desorption and reaction of adsorbed ammonia indicated the formation of NHx species at the surface, and that this species is the precursor for the formation of nitrogen and nitrogen oxides. Pt/CuO/Al2O3 is the most active sample for nitrogen formation. In experiments pulsing alternately ammonia and oxygen it is observed that nitrogen is predominantly formed at the oxygen pulse on Pt/CuO/Al2O3 and at the ammonia pulse on Pt/Al2O3 and CuO/Al2O3. Results obtained in the alternate pulsing of ammonia and NO over preoxidised samples exclude NO from being a reaction intermediate for nitrogen formation. The performance of Pt/Al2O3 and CuO/Al2O3 in NH3-NO2 alternate pulsing reveals that nitrogen formation on these two samples occurs by formed NO2 reacts with an NHchi species. Over Pt/CuO/Al2O3, on the other hand, most nitrogen is formed by the direct oxidation and combination of two NHchi species using adsorbed oxygen. The data obtained suggests that the reaction takes place at the Pt/CuO phase boundary between NHx species being adsorbed on CuO and oxygen on Pt sites. (C) 2004 Elsevier Ltd. All rights reserved.