The ammonia oxidation reaction on Pt{100} has been investigated over the temperature range 300-800 K, using molecular beams under UHV conditions. The reaction is biphasic, with N-2 being the major product below 600 K and NO being the major product above 600 K, It is found that product selectivity can be controlled by varying the beam composition as well as by varying the surface temperature. The efficiency of the reaction to NO can be significantly increased by preadsorption of oxygen on the crystal. Coadsorption and isothermal experiments translate the beam composition dependence into a surface oxygen coverage dependence, with high oxygen coverages resulting in the suppression of N-2 production. N-2 is believed to be produced mainly from the dissociation of NO produced by oxidation of adsorbed NH3. The observed oxygen coverage dependence of product formation is explained by a sharp fall in the heat of adsorption of the dissociated N(a) and O(a) with increasing O(a) coverage. At high surface oxygen coverages the suppression of N-2 production arises from the resulting inhibition of NO dissociation. The observed surface temperature dependence of product formation is explained by competition between NO desorption and NO dissociation.