The poly(3,4-ethylenedioxythiophene) (PEDOT) films prepared in an aqueous sodium dodecyl sulfate and lithium tetrafluoroborate electrolyte solution were investigated. It was shown that the PEDOT films prepared in this medium exhibit novel electrochemical and optical properties. Cyclic voltammetric results indicate that the dodecyl sulfate (DS-) anions are trapped in the polymer matrix when PEDOT is deposited onto the gold substrate. The larger DS- anions are immobile and stay in the polymer during a redox process, whereas cation movement dominates the charge compensation, moving in and out of the polymer matrix, when the PEDOT film is redox-cycled. The infrared and X-ray photoelectron spectroscopic results further confirmed the incorporation of DS- into the PEDOT film. Surface plasmon resonance was used to investigate the film growth and the optical properties of PEDOT films upon doping and dedoping at two laser wavelengths. With the film growing, an increase in doping level as well as an increase in refractive index was detected. The resonance curves change dramatically when the PEDOT film is switched to different doping states, suggesting important changes on the film dielectric constant and electrochromic property.