The effect of beta-CD and alpha-CD on the electrochemical behaviour of H(2)A and HA(-) on platinum is studied. The adsorption of beta-CD on this electrode is demonstrated and proved to be dependent on the base electrolyte composition. The maximum adsorption coverage was reached in phosphate solution at pH 6.95. The homogeneous H(2)A-beta-CD complex formation produced a decrease in the oxidation current and a positive shift in the oxidation peak potential. These effects are predominant in acid solutions. In neutral solutions the opposite behaviour is observed, i.e. an increase in the oxidation current at lower oxidation potentials, The presence of a parallel oxidation route for the vitamin involving strongly adsorbed CO residues is considered, and evidence for a decrease in COad. in the presence of beta-CD was given by variations in hydrogen adsorption charges. This fact, more important in neutral solutions, must be responsible for the catalytic effect observed. alpha-CD was not adsorbed, neither did it modify the electro-oxidation behaviour of H(2)A and HA(-).