In this study, we use a novel method for simultaneously measuring the optical and electrochemical proper-ties of layer-by-layer self-assembled conducting ultrathin polymer films. We employed the recently developed combination of in situ surface plasmon resonance spectroscopy (SPS) and surface plasmon field-enhanced light scattering (SPFELS) with an electrochemical method (cyclic voltammetry). Polyaniline (PANI) and sulfonated polyaniline (SPANI) were used as the polycation and the polyanion for the conducting layer-by-layer film assemblies. The doping-dedoping process of the PANI/SPANI at different film thicknesses was investigated by electrochemical-SPS/SPFELS. The potential cycling resulted in characteristic oscillations that were sensitively monitored with this technique. We were able to obtain in situ information on dielectric constant changes of the PANI/SPANI layer-by-layer films with respect to film thickness and their corresponding morphology transitions during the electrochemical process. Thus, this experimental approach allows for the simultaneous elucidation of optical and electrochemical properties of conducting polymers in thin film formats.