The electrodeposition of polysiloxane precursor polymers to form cross-linked conjugated polythiophene ultrathin films is described. The precursor polymer contains a polysiloxane backbone with pendant electroactive thiophene monomer units synthesized via a hydrosilylation reaction. The thiophene units were electropolymerized by cyclic voltammetry. Investigations were made on the oxidation and reduction of the thiophene monomer and the polymers formed. A uniformly smooth layer-by-layer growth of the film was observed by repeated cycling based on the cyclic voltammogram, surface plasmon spectroscopy (SPS), and the atomic force microscopy images. The morphology of the film changed with increasing number of cycles being transformed from a relatively globular to a more membranelike morphology. Correlation was also made on the morphological changes with the electropolymerization process and the extent of crosslinking. UV-vis spectra showed clear evidence of polythiophene structure formation in the cross-linked polymer. SPS confirmed that the thickness of the deposited polymer film is easily controlled by the number of electrochemical potential cycles.