Density functional theory simulations were used to study the mechanism of ammonia oxidation over Pt(100). The conversion of NH₃ leading to NH_x intermediates upon reaction with adsorbed oxygen-containing species and ultimately forming reaction products (NO, N₂O, N₂, and H₂O), have been systematically computed. The reaction proceeds via an imide mechanism, while classical mechanisms postulating nitroxyl and hydroxylamine as reaction intermediates may be excluded. The barriers of oxidative NH₃ dehydrogenation over Pt(100) are drastically decreased with respect to the nonoxidative dehydrogenation, particularly when the number of hydrogen atoms in the NH_x fragment is decreased. Ammonia activation and subsequent NH_x dehydrogenation steps are greatly favored by Oads with respect to OHads on Pt(100). This differs from calculations on Pt(111) due to the metal sharing effect and to the lower stability of adsorbed hydroxyl in the latter facet. Nitrogen-containing products are formed by recombination of chemisorbed N with N (N₂), O (NO), and NO (N₂O). Water is formed via recombination of adsorbed OH, regenerating an active O. The O-mediated abstraction of the first proton of NH₃ is the slowest dehydrogenation step, whereas NO desorption determines the rate of the overall process. Rate coefficients of the elementary steps involved in the mechanism have been calculated, enabling a microkinetic analysis of the reaction. Our simplified model predicts reasonably well the product distribution obtained experimentally at different temperature, time, and NH₃/O₂ ratio.