The electrocatalytic effect of the conducting poly(3-methylthiophene) film (3-PMT) electrode on the redox behavior of catechol compounds has been examined. The nature of the electrode material substrate had no effect on the peak potentials in the cyclic voltammograms (CVs) of different redox systems of biological interest : p-aminophenol, dopamine, catechol and ascorbic acid. The nature of the heteroatom of 3-PMT, polypyrrole, polyfuran and polyanaline had a very large effect on their electrokinetic properties. The adsorption of molybdenum ion results in the blocking of ’the active sites’ at the interface, as indicated by the disappearance of the electrochemical activity of the 3-PMT film electrode. The surface examination by X-ray photoelectron spectroscopy (XPS) of the molybdenum exposed polymer suggests the possibility of a chemical bonding between the metal ions and the sulfur heteroatom of the polymer surface. The ’apparent’ diffusion coefficients D-app were determined from the effect of the scan rate dependency of CVs of the catechol, etc. The results showed that the electron transfer is predominantly diffusion controlled at the polymer / solution interface and probably involves the sulfur heteroatom specifically.