A systematic analysis for electrochemical polymerization of aniline, o-toluidine (OTO), and m-toluidine (MTO) ternary components on an indium-tin oxide (ITO) electrode was performed. Using an experimental design of mixture, empirical models are fitted and plotted as contour diagrams, which facilitate revealing the synergestic/antagonistic effects between these mixed aniline derivatives. Copolymer produced by electropolymerization of these aniline derivatives in 1 M HCl aqueous solution was investigated by cyclic voltammetry (CV), UV-visible spectroelectrochemical study, and scanning electron spectroscopy (SEM). The cyclic voltammogram of the copolymer growth is significantly affected by the composition of monomers. As shown by CV, the mechanism and the rate of the copolymer growth are significantly affected by the composition and concentration of monomers. The SEM results show that a fiber-like form is changed as a consequence of incorporation of OTO/MTO units into the polymer structure. UV-visible spectrum of the copolymer showed three optical transitions at lambda(max) = 300, 440, and 800 nm for different applied potentials. At isosbestic points of ca. 350 and 480 nm, a fixed stoichiometric ratio of oxidized to reduced form exists when the polymer film is oxidized successively by applying potentials from 0 to 1 V vs Ag/AgCl. (c) 2005 The Electrochemical Society.