A novel conducting polymer, poly(aniline-co-N-methylthionine), is synthesized in a mildly acidic aqueous solution using cyclic voltammetry. The rate of copolymerization and the electrochemical properties of the obtained copolymers are substantially affected by pH, upper potential limit and comonomer concentration ratio. The copolymer possesses a high electrochemical activity in an aqueous solution up to pH 10.0. The copolymer formed on an indium-tin oxide glass electrode exhibits a black-to-transmissive electrochromism under relatively low potentials. The results from the Fourier transformed infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) of the copolymer show that NMTh units and Cl- ions were incorporated into the copolymer backbone during the electrosynthesis. The scanning electron microscopy (SEM) micrograph demonstrates that the copolymer film is composed of polymeric flower-like microparticles with an average diameter of 1 mu m. (C) 2015 The Electrochemical Society. All rights reserved.