The mechanism electro-oxidation of cysteine, cysteinesulfinic acid and cysteic acid in perchloric acid solutions has been studied using cyclic voltammetry. All the compounds investigated have been found to be chemisorbed on a polycrystalline gold electrode, and oxidized with four, two or one electron respectively. The water molecule coadsorbed with cysteine participates in the cysteine one-electron oxidation with a lower energy requirement, whereas bulk water is involved in multiple electron transfer reactions at the completely covered electrode surface. A strong inhibiting effect of chloride anions on the cysteine oxidation has been observed at the partially covered electrode surface. The effect of the adsorbate surface coverage on the reaction mechanism in anodically catalyzed reactions involving oxygen transfer is discussed.