The probe beam deflection technique (PBD) has been successfully used to detect in situ the influence of electrochemical mode and conditions on the initial stages of poly-3-methylthiophene formation. The interpretation of the results is coherent with the previously reported oligomer formation in solution prior to deposition. The effect of the initial polymerization steps in the subsequent stages of the polymer growth has also been investigated, analysing the redox behaviours along with the correspondent deflectograms obtained for thin films synthesized under different deposition charges. The data indicate that up to a given thickness the polymer responses reflect the nature of the first deposited layer. The electropolymerization behaviour of methylthiophene has also been contrasted to that of pyrrole under the same experimental conditions (solvent, monomer concentration, anionic and cationic components of the electrolyte). The results clearly reveal an earlier deposition process and that oligomer formation, if any, is not determinant for polypyrrole deposition.