The electrochemical synthesis of polymelatonin was studied on bare electrodes and the poly(3,4-ethylenedioxythiophene) (PEDOT) modified electrodes from the aqueous acidic solution through the cyclic voltammetry method. It was found that electropolymerization was very much dependent on the experimental conditions such as the monomer oxidation potential, the PEDOT film thickness, and also the monomer concentrations. The electrochemical quartz crystal microbalance (EQCM) method was used to study the polymer growth mechanism on the Au and Au/PEDOT-modified electrodes. The EQCM results confirmed that enhancement in the polymer peak currents of polymelatonin on the PEDOT-modified electrode was due to the hydrophobic entrapment of the monomer molecules within the PEDOT film. The UV-visible spectroelectrochemical results confirmed the oxidation of the indole ring of the monomer. The electrochemical oxidations of ascorbic acid (AA) and sodium sulfite were performed on the glassy carbon (GC) and GC/PEDOT/polymelatonin film-modified electrodes. The polymer-modified electrode was found suitable for effective catalysis of the oxidation reaction of AA and SO32-. The catalytic current was found increased with increase of AA and SO32- concentrations and resulted in correlation coefficients of 0.9998 and 0.9989, respectively. Based on the UV-visible and cyclic voltammetry results, a mechanism for electropolymerization has been proposed. (c) 2005 The Electrochemical Society.