The electrochemical doping and undoping processes of poly-(3,3′′-didodecyl-2,2′:5′,2′′terthiophene) (poly(33′′DDTT)) were studied by scanning electrochemical microscopy (SECM) to characterize the electron transfer of the polymer with a redox mediator in solution when the polymer was at different oxidation levels. SECM showed that electron exchange processes of oxidized poly(33′′DDTT) were localized at the polymer/solution interface rather than inside the film. Poly(33′′DDTT) in the neutral state did not allow the permeation of redox species to the underlying metal and behaved like a completely passivating film. A modified expression for the effective electron-transfer rate constant, keff, was obtained by grouping the terms concerned with charge transport across the polymeric film, and the values of keff were calculated from SECM approach curves. The values of keff for the electron transfer between poly(33′′DDTT) film and methyl viologen (MV+/2+) as the redox couple ranged between 10-5 and 10-1 cm s-1, depending on polymer thickness, substrate potential, and MV concentration. Under the proper experimental conditions, keff could be directly correlated with the conductance of the polymer film, where poly(33′′DDTT) thin films in the conducting state showed a metallic character.