The electrochemical polymerization of a Vitamin B-12 derivative containing a pyrrole group in a peripheral alkyl chain is achieved on Pt under potentiodynamic conditions. The process efficiency depends critically on the range of the cathodic potential applied, supporting the view that the release of one CN- group upon the Co-complex reduction promotes the adsorption of the modified complex prior to its electropolymerization. AFM analysis revealed the common characteristics of a conducting polymer in what concerns to its morphology. In situ ellipsometric measurements have been used to probe changes in the polymer films induced by the applied potential. The data reveal marked structural reorganisations upon metallic centre redox conversions; polymer oxidation originates a density decrease and a very small increase in conductivity. The low conducting character, indicative of films formed by short chains, is also retrieved from the obtained values for polymer refractive index. However, the modified electrodes present interesting electrocatalytic properties as shown by the behaviour towards the ORR. (C) 2004 Elsevier Ltd. All rights reserved.