The effect of oxygen on the activity and selectivity of the NO reduction by NH3 over Pt-Rh (95-5 wt.%) alloy gauze at 1023-1073 K has been investigated. To this end, the Temporal Analysis of Products (TAP) reactor in combination with isotopic tracers was applied. Single pulse experiments evidenced the rapid activation of NH3 over the catalyst surface covered by adsorbed oxygen. Contrarily, NO requires an essentially reduced surface in order to be dissociated. Adsorbed oxygen species in the O-2-pretreated gauze accelerate the reaction of NH3 with NO with respect to the as-received gauze. N-2 was the main reaction product and traces of N2O were comparatively formed (N2O/N-2 similar to 10(-3)). Pulsing of an equimolar O-2-(NH3)-N-15-NO mixture over the Pt-Rh gauze mainly produces 15 NO, while the formation of N2 is largely suppressed. The selectivity to (NO)-N-15 and N2O in the ternary O-2-(NH3)-N-15-NO system diminished upon decreasing the O-2/((NH3)-N-15 + NO) ratio, in favor of N-2. This ratio was qualitatively varied by changing the time delay between O-2 and (NH3)-N-15-NO in sequential pulse experiments. Our results indicate that the NH3 oxidation by O-2 to NO is much faster than the NO reduction by NH3 at similar concentrations of oxygen and nitric oxide. This explains the low production of N-2 and N2O in ammonia burners within nitric acid manufacture. (c) 2005 Elsevier B.V. All rights reserved.