Indole/aniline copolymers were synthesized electrochemically and the polymerization mechanism was investigated via in-situ simultaneous electrochemical electron spin resonance (SEESR) spectroscopy. The Radical cationic intermediates of substituted indole and aniline were detected during the electrochemical oxidation. The resulting ESR-spectra with hyperfine splitting were simulated and radicalic structures were identified. It was suggested that the polymerization occurs over positions C-3 and C-6 of indole molecule and over the nitrogen and para-position of aniline. Furthermore, aniline, indole and aniline/indole copolymers were investigated depending on applied potential via electrochemical impedance spectroscopy (EIS). (c) 2008 Wiley Periodicals, Inc.