Electrochemical studies of a series of regioregular 3-octylthiophene oligomers with 5-8 thiophene rings, chlorine end-capped at one alpha terminal position, have been conducted in solution and in thin film state. Results indicate gradual coalescing of the oxidation potentials of neutral and radical cation oligomer molecule, indicative of coming close to sufficient length of the molecule to allow for two separate radical cations to exist along its chain. With increasing number of thiophene rings in the molecule, further quasi-reversible oxidation reactions upon which trication and tetracation were formed have been observed at ever decreasing potentials. It was also observed that the oxidation of radical cations comes about more easily than that of electron-paired cations, the latter process being strongly dependent on the length of oligothiophene chain. Thin film electrochemistry revealed that the processes taking place during redox reactions differ from those in solution. Sharp oxidation peaks were observed which were attributed to conformational changes taking place in the oligomer molecules upon transition to the conductive form, alongside the film's oxidative doping process. To shed some light on the issue of charge carriers appearing in the film upon its oxidation, supplemental ESR spectroelectrochemical measurements were carried out and confirmed that, radical cations are generated in the film. (C) 2003 Elsevier Science Ltd. All rights reserved.