The electrochemical behaviour of croconic acid at platinum single crystal electrodes with Pt(100) and Pt(2n-1,1,1) orientations has been studied in 0.5 M H2SO4 by cyclic voltammetry. Experiments with croconic acid in solution show two main processes : the oxidation of surface poisons at around O.8 V, and the oxidation of the acid, between 0.9 and 1.25 V. While the currents involved in the first process tend to increase with the terrace width, the process at higher potentials gives maximum current density for short terraces (3-6 atoms width). Adsorption experiments have shown that, although strongly adsorbed species are formed from croconic acid even under open circuit conditions, the application of a polarization significantly increases the amount of poison formed. For each surface, maximum amounts of irreversibly adsorbed species are formed at potentials around 0.4-0.5 V. The greatest amount of strongly adsorbed species is obtained with the Pt (100) electrode, for which adsorbed CO seems to be the only adsorbate. For stepped surfaces, some other intermediates may also be formed.