The electrochemical oxidation of ammonia to dinitrogen is a model reaction for the electrocatalysis of the nitrogen cycle, as it can contribute to the understanding of the making/breaking of N–N, N–O, or N–H bonds. Moreover, it can be used as the anode reaction in ammonia electrolyzers for H 2 production or in ammonia fuel cells. We study here the reaction on the N 2 -forming Pt(1 0 0) electrode using a combination of electrochemical methods, product characterization and computational methods, and suggest a mechanism that is compatible with the experimental and theoretical findings. We propose that N 2 is formed via an ∗ NH + ∗ NH coupling step, in accordance with the Gerischer-Mauerer mechanism. Other N–N bond-forming steps are considered less likely based on either their unfavourable energetics or the low coverage of the necessary monomers. The N–N coupling is inhibited by strongly adsorbed ∗ N and ∗ NO species, which are formed by further oxidation of ∗ NH.