A novel ABA triblock copolymer containing electroactive tetraaniline [(ANI)(4)] and poly(ethylene oxide) (PEO600, M-n = 600), (ANI)(4)-b-PEO600-b-(ANI)(4), was synthesized by coupling tetraaniline and PEO600 with tolylene 2,4-diisocyanate. FTIR, NMR, and UV-vis spectroscopy were combined to characterize the chemical structure of (ANI)(4)-b-PEO600-b-(ANI)(4). The electrochemical properties, self-assembly, and acidity response of copolymer in aqueous solution were investigated by cyclic voltammetry, electron microscopy, and dynamic light scattering. Different from pure tetraaniline and polyaniline, the triblock copolymer in 1.0 M sulfuric acid solution only exhibits one oxidation peak in cyclic voltammetry. In a neutral aqueous solution, the triblock copolymer self-assembled into vesicles with diameter of about 258 nm. Upon acidification with HCl, the size of the vesicles increases to 471 nm and 1.19 mu m when the concentration of HCl changes to 10(-3) and 10(-1) M, respectively. With addition of aqueous 1.0 M HCl to the triblock copolymer solution in THF, hollow spheres and bowl-like aggregates were obtained. A bilayer model was proposed for the vesicle formation, and the mechanism of acidity response was discussed.