We have performed density functional theory simulations applied to slabs to study the dehydrogenation of ammonia on the surface of clean and atomic oxygen or hydroxyl precovered Au(111). Ammonia does not dissociate unimolecularly on the surface and needs a proton scavenger to drive decomposition. O atoms or hydroxyl groups on the surface are basic enough to attract the first H atom from the molecule and are also needed in the resting dehydrogenation steps. Recombination steps to form N₂ and NO are hindered by barriers lower than those on Pt or Rh(111) due to the low interaction energies of molecular moieties with the Au(111) surface. The barrier for N₂ formation is in any case lower than that of NO in agreement with the high selectivity observed toward N₂. Finally, coverage effects are discussed for the competing reactions.