Indole and pyrrole solutions have been mixed in different ratios and oxidized under anodic conditions on Pt surface in acetonitrile/0.1 N LiClO4 solution. The electrocopolymerization mechanism and the electrochemical properties of synthesized copolymers have been investigated in detail. Current-potential diagrams of polypyrrole (PPy), polyindole (PIn), and the resulting copolymers have been recorded. In-situ resistance-potential measurements of synthesized copolymers have been carried out under atmospheric conditions. A significant variability have been observed in the conductivity of the copolymers, depending on applied potentials. Electrochemical behaviour of homo-polymers and copolymers have been studied at different temperature conditions. Simultaneous-electrochemical-electron-spin-resonance (SEESR) spectroscopy has been utilized for mechanistical investigation. (C) 2002 Elsevier Science B.V. All rights reserved.