The oxidation of ammonia on platinized platinum has been studied with cyclic voltammetry and differential electrochemical mass spectrometry (DEMS). These techniques show the surface to be highly covered with adsorbates during the selective oxidation of ammonia to N-2 at potentials where platinum is free of oxides. These adsorbates are inactive in the formation of N-2 and consist of NHx, probably N-ads, whereas no NO adsorbates are present among these adspecies. These adsorbates remain present on the surface after exchange of the ammonia solution for base electrolyte and in a negatively directed potential scan N-2 and NH3 are formed. When this potential scan is interrupted by holding the potential at 0.55 V the current reverses from negative to positive, being accompanied by N-2 formation. These data support a mechanism in which NHx species, proposedly NHads, are the active intermediates and N-ads acts as a poison.