The electrochemical oxidative polymerization of 3-methylthiophene (3MT) on platinum electrodes is favoured by the presence of previously deposited polyterthiophene (PTT) films. Cyclic voltammetry CV and single sweep voltammetry SSV were used to investigate the electrocatalytic process. The CV rate of polymer growth and the SSV electrocatalytic current increase linearly with the amount of predeposited PTT. The results rule out the previously invoked electrocatalytic promotion of 3MT polymerization by terthiophene radical cations in solution. SSV analysis with other thiophene-based polymer films (e.g. 3- and 3,4-alkylsubstituted polythiophenes) and with other thiophene-based monomers (e.g. 3,4-ethylenedioxythiophene) has shown that the electrocatalytic effect is of a general nature. The formation of a charge transfer complex between the thiophene-based monomer and the oxidized thiophene-based polymer is invoked to account for the catalytic action. The results account for the previous large and unexplained body of experimental evidence of electrocatalytic effects in the electro oxidative synthesis of conducting polymers. (C) 2004 Elsevier B.V. All rights reserved.