Ammonia oxidation was studied in situ on Ir(110) and Ir(111) under low-pressure (similar to 10(-7) mbar) conditions. NH3 does not dissociate on a flat Ir(111) surface, but surface defects and O-ad facilitate NH3ad decomposition. High-energy resolution fast XPS measurements were used to monitor the surface coverage during reaction on Ir(110), and temperature-programmed reaction measurements were applied to reveal the gas phase reaction products during reaction. The steady-state NH3 oxidation reaction starts between 350 and 500 K on both surfaces, which also show similar selectivity. Below 600 K, N-2 and H2O are the principal reaction products. Above 600 K, the selectivity changes to NO and H2O, with the exact temperature depending on the NH3:O-2 pressure ratio. The surface population changes from NHad/N-ad to Oad around 500 K, about 200 K lower than the selectivity change from N-2 to NO observed in the gas phase. This behavior can be explained by considering the activation energies for N-2 and NOad formation. We present a model to explain why Ir is more selective toward N-2 than Pt. (c) 2005 Elsevier Inc. All rights reserved.