The in situ electrochemical and dielectric film properties of monomer co-electropolymerization with a CPN precursor was investigated. A polysiloxane/polythiophene precursor was electropolymerized with thiophene monomer at different composition ratios using cyclic voltammetry. This was investigated in situ by EC-SPR spectroscopy and EC-SPFELS. The characteristic oscillations corresponded to distinct transitions in film deposition. The resonance coupling of the SPR decreased with higher thiophene monomer content. Increasing the amount of thiophene monomer increased the RMS roughness of the film as observed by AFM, which corresponded to an increase in scattered light intensity with EC-SPFELS.