The reactions of oxygen with ammonia have been studied over the Pt(S) — 12(111) × (111) surface for pressures between 10 −10 and 10 −8 Torr over the temperature range −75 to 600 °C. These steady-state reaction studies were performed in an ultrahigh vacuum system equipped with low-energy electron diffraction for determination of surface structures, Auger electron spectroscopy (AES) for surface composition determinations, and mass spectroscopy for determination of gas-phase compositions. The oxidation of ammonia proceeds rapidly above 150 °C on the Pt(S) — 12(111) × (111) surface; the products observed are molecular nitrogen, nitric oxide, and water. For a stoichiometric mixture of ammonia and molecular oxygen, nitrogen formation predominates at low temperature (150 to 400 °C) while nitric oxide formation predominates above 400 °C. Kinetic studies done with excess ammonia reveal that the nitrogen formation rate is first order in gas-phase oxygen pressure but saturates with increasing ammonia pressure. Under these conditions the surface is primarily covered with nitrogen-containing species. Kinetic studies done in excess oxygen indicate that nitric oxide formation is first order in ammonia pressure but saturates with increasing adsorbed oxygen pressure. Under these conditions the surface is covered with adsorbed oxygen (as determined by AES). Thermal desorption of oxygen adsorbed on the platinum surface during ammonia oxidation indicates that the oxygen is present as oxygen atoms bound to the step edge sites. The observations that the reaction rates for both branches of this reaction saturate with respect to increasing reactant pressure just as the step edge sites become saturated indicate that the step edge sites play a predominant role in both reaction branches.