Self-assembled monolayers of poly(3-alkylthiophene) on gold electrodes are examined by cyclic voltammetry in solutions containing electroactive species. Two well-separated electron transfer processes, namely, electron tunneling through the monolayer and electron exchange at pinholes (defects) of the monolayer are observed. The voltammetric responses of the pinhole electron transfer process take place around the standard potential of the electroactive species and resemble those of a nanoelectrode ensemble of independent individual nanoelectrodes. The voltammetric characteristics of the electron tunneling agree well with predictions of the Marcus theory. Satisfactory values of tunneling coefficient, standard rate constant and organization energy are derived from the voltammetric data.