The catalytic electrochemical oxidation of ammonia is a structure-sensitive reaction that will potentially play a role in future energy systems, including portable fuel cells and the elimination of harmful pollutants. Platinum is an ideal catalyst for this reaction because of its selectivity toward the formation of nitrogen gas. The Pt (100) facet shows much faster ammonia electroxidation than other facets of Pt, however the elementary reaction steps responsible for this phenomenon are not understood. Density functional theory (DFT) calculations are used to determine elementary reaction thermodynamics and kinetics. Absorbed NH2* formation is rapid and this intermediate is very stable on Pt (100). High coverage NH2* binds at atop sites, enabling favorable N-N bond formation. The faster rate of ammonia oxidation on Pt (100) results from the low barrier of N2H4* formation resulting from NH2* dimerization at high coverage. Understanding the reactivity of ammonia oxidation of Pt (100) can aid in electrochemical reaction mechanism development and catalyst design.