The electropolymerization of 5-cyanoindole in acetonitrile leads to the formation and deposition of a cyclic trimer. This trimer then undergoes further oxidation on the electrode surface to form polymeric species. The trimer and polymers can be separated by their differential solubility, and they have been structurally characterized. The structure of the trimer is analogous to that found when indole-5-carboxylic acid is polymerized, indicating that the trimer is formed by a similar mechanism. Electrochemical studies indicate that both free trimer and the trimer centres in the polymer are less easily oxidized than indole-5-carboxylic acid; this can be attributed to the increased electronegativity of the 5-cyano substituent which increases the energy of the oxidized form of the redox centre.