NH3 does not dissociate on a clean Ir(1 1 1) surface, but dissociation can be induced by radiation, which yields all possible NHxad species. NHad is the most stable and it is the only intermediate found at room temperature. NHad decomposes between 350 and 500 K, yielding NH3 (g) and N-ad, which desorbs as N-2 between 550 and 700 K. Adsorbed oxygen atoms induce NH3 dissociation between 300 and 400 K, forming NHad and H2O. NHad decomposes further between 350 and 450 K. forming N-ad and H2O. Measurements of the surface composition during ammonia oxidation showed a mismatch between the change of the surface coverage (from N-ad to O-ad-dominated) and that in the gas phase (from N-2 to NO). This is explained by a higher barrier for NO (g) formation as compared to N-2 formation on Ir(1 1 1). On Pt(1 1 1) the difference in barrier height for N-2 vs. NO formation is smaller, which explains why Pt is more selective to NO than Ir. (C) 2010 Elsevier B.V. All rights reserved.