Thin and thick poly(3-methylthiophene) (PMT) films were electropolymerised on platinum electrodes and characterised by cyclic voltammetry and coulometry combined with optical measurements, scanning electron microscopy (SEM) and gravimetrical measurements. The polymerisation was carried out in the system: Pt/propylene carbonate + 0.030 M TBAPF(6) + 0.20 M 3-methylthiophene at a constant current density (i = 1.0 mA cm(-2)) in deaerated solution. Thin film electrodes (0.07 mu m) show a quasi reversible oxidation/reduction behaviour whereas at thick film electrodes (8.1 mu m) the redox behaviour is impeded by transport processes. Gravimetrical measurements show that the current efficiency eta of the electropolymerisation process is eta = 0.84, due to the formation of soluble oligomers and overoxidation of the PMT film. The PMT film can be oxidised up to a degree of oxidation of alpha(r) = 0.27. Tn contrast to the generally assumed linear growth models, the thickness of the polymer film increases overproportional with increasing overall electrolysis charge. A correlation between the degree of oxidation and the electronic and ionic conductivities of PMT were found, from which the mobilities and diffusivities of electrons and ions were determined.