Poly 3,4-(ethylenedioxythiophene) (PEDOT) films electropolymerized from an aqueous micellar solution containing sodium dioctyl sulfosuccinate and the monomer were functionalized with 1-fluoro-2-nitro-4-azidobenzene (FNAB) molecules by a photochemical nitrene insertion reaction. The variation in redox activity and the changes in the charge transfer and diffusion (through bulk) behavior of the functionalized and the non-functionalized PEDOT films have been followed by electrochemical impedance spectroscopy and cyclic voltammetry. While the functionalized film allows a reversible insertion and extraction of guest cations and anions, the non-functionalized film is capable of exchanging only anions. The higher level of oxidation attained in the functionalized film is also reflected in the longer diffusion length (I-D) observed for the ions in this film. In both films the barrier to charge transfer is resistive rather than capacitive. Both charge transfer and diffusion resistance (R-CT and R-D) are lower for the functionalized film, a consequence of a higher surface roughness and a more nodular morphology and therefore higher optical contrast and faster color-bleach kinetics are achieved in this film. For the functionalized and the non-functionalized films, both R-CT and R-D are greatly enhanced during reduction than for oxidation. In particular, in the low frequency regime, the hindered diffusion-controlled extraction of anions from the bulk of the film is also evident from the larger R-D as compared to R-CT and the difference in their magnitudes is more pronounced for the functionalized film thus confirming that functionalization is a useful method for controlling the redox response of conducting polymer films. (C) 2008 Elsevier Ltd. All rights reserved.