Ammonia oxidation reaction pathways on high surface area silver powder have been studied by TPD, temperature programmed reaction FT-Raman, and transient as well as steady-state ammonia oxidation experiments. NO was found to be the main reaction intermediate yielding N2O as well as N-2. NO could be formed even at room temperature and when oxidized to NOx became adsorbed to the silver surface, thus blocking the active sites for oxygen dissociation. The dissociation of oxygen is thus believed to be the rate-controlling step for ammonia oxidation. The selectivity for N-2, N2O, and NO is mainly determined by the surface oxygen coverage and by reaction temperature. The adsorbed NOx and N2Ox species are actually inhibitors for ammonia oxidation but these adsorbed species lower the surface oxygen coverage. Hence the selectivity for nitrogen is improved with increasing amounts of these adsorbed species.