Applying the potential step method, the effects of potential, monomer concentration, nature and concentration of electrolyte on the nucleation and growth processes of polythiophene on a Pt electrode in CH2Cl2 were studied. The current-time transients obtained were fitted using a mathematical equation that considers three contributions corresponding to 2D instantaneous and 3D instantaneous nucleation, both charge-transfer-controlled mechanisms, and 3D progressive nucleation with a diffusion-controlled mechanism. In any condition the principal contribution was always 3D instantaneous nucleation. However, the contribution of 2D instantaneous nucleation is important in the first stages of the nucleation process, indicating that the polymeric deposit initiates by the formation of a two-dimensional film. The contribution of 3D progressive nucleation appears at longer times and it is favoured with increasing monomer concentration. From the general nucleation and growth equation proposed, a relationship for the film thickness variation with time is obtained. Also, a general scheme of the nucleation and growth mechanism for poly(thiophene) is given.