Electrochemical oxidation of terthiophenes leads to polymers forming electroactive layers on the electrode surface. Application of spectroelectrochernical methods and special measurement procedure make it possible to show that intermediate products are formed during polymerization. Detailed spectroscopic analyses reveal the presence of terthiophene dimer, trimer and tetramer which allow to propose a mechanism of polymerization process. (c) 2007 Elsevier Ltd. All rights reserved.