Product species present in the diffusion layer during electropolymerization of thiophene in acetonitrile were investigated by UV-visible absorption spectroscopic methods. Absorbances from soluble oligomeric species were observed at 290, 390, 570, 670, 830 and 990 nm using a platinum-mesh as well as a long path cell with a platinum-foil as working electrode. The absorbances were assigned tentatively to neutral, cationic and dicationic oligomers of thiophene with less than six monomeric units. The method of diffusion layer imaging was used to obtain information on the formation and depletion of the various species in solution from the recorded absorbance-distance profiles. The diffusion coefficients for soluble oligomeric species were determined at three wavelengths yielding D337 = 0.7(+/-0.4) x 10(-5) cm2 s-1, D544 = 0.8(+/- 0.2) x 10(-5) cm2 s-1 and D670 = 0.5 (+/- 0.2) x 10(-5) cm2 s-1, respectively.