An understanding of the deactivation mechanism of Pt electrocatalysis towards the ammonia oxidation reaction (AOR) to dinitrogen is key to the successful introduction of the nitrogen energy cycle, which can be used as a source of hydrogen fuels. Herein, we study AOR electrocatalysis on NOx-adsorbed polycrystalline Pt electrodes, i.e. NO, NO2− and NO3−, and find that all the NOx species can significantly reduce the catalytic activity of Pt. Combined stationary/transient voltammograms reveal that the poisonous NOx species are produced by NH3 oxidation on the bare Pt surface, but the adsorbed NOx species can be transformed to N2 by the Langmuir–Hinshelwood mechanism with *NO as a key intermediate. Display omitted •NO, NO2− and NO3− reduce the catalytic activity of Pt in NH3 oxidation reaction.•The NOx species are produced by NH3 oxidation on Pt at high potentials.•NOx species accumulate on Pt surface at high potentials.•The NOx species can be transformed to N2 via the Langmuir–Hinshelwood mechanism.