The electrochemical behaviour of croconic acid (CRA) on Pt(111) electrodes has been studied in acid medium by cyclic voltammetry. Its oxidation proceeds via a dual path mechanism. The direct oxidation path takes place at potentials higher than 0.9 V, probably involving incipient surface oxide formation. The formation of strongly adsorbed species upon CRA adsorption is influenced by the concentration of CRA and the adsorption potential. At open circuit, only COads is formed, while at controlled potentials lower than 0.40 V, other adsorbates, probably with more than one carbon atom, exist. These are oxidized to CO in the range 0.4-0.6 V. ?he activity for poison formation at open circuit is much lower for concentrations higher than 1 mM. FTIR experiments clearly indicate that carbon monoxide is not formed at potentials lower than 0.40 V. Linearly- and bridge-bonded CO adspecies appear at potentials higher than 0.40 V, which are oxidized to CO2 before the direct oxidation of CRA begins. A band at 1780 cm(-1) indicates the presence of leuconic acid at potentials between 0.95 and 1.22 V as a product of the latter process, At higher potentials, CRA is completely oxidized to CO2.