The electro-oxidation and -reduction of 0.05 M ammonia, 0.01 M hydroxylamine, 0.01 M sodium nitrite and 0.05 M sodium nitrate in 0.5 M KOH at Pt-black electrodes has been investigated using a combination of cyclic voltammetry with on-line MS analysis of volatile products (DEMS). All compounds investigated in this study form adsorbates with very similar properties. These adsorbates can be reduced to ammonia and oxidised to nitrogen. Ammonia is adsorbed anodically, while nitrite and nitrate are adsorbed cathodically. In contrast to the adsorbates of the other compounds, adsorbed nitrite can also be reduced to nitrogen oxides in addition to ammonia. The combination of cyclic voltammetry and on-line MS proves that the adsorbates do not consist solely of triple bonded nitrogen. Ammonia and hydroxylamine also form nitrogen oxides in bulk oxidation processes. For the first time, a reaction mechanism which takes into account the interconversion of nitrogen compounds and adsorbate processes is discussed.