Poly(aniline-co-3-amino-4-hydroxybenzoic acid) was synthesized using cyclic voltammetry. The monomer concentration ratio and applied potential strongly affect the copolymerization rate and electrochemical properties of the copolymer. The results from the infrared spectrum (IR) and X-ray photoelectron spectroscopy (XPS) of the copolymer demonstrate that 3-amino-4-hydroxybenzoic acid units are incorporated into the copolymer chain and that SO4 2− ions can be doped into the copolymer film during the electrochemical copolymerization. The scanning electron microscopy (SEM) micrograph proves that the copolymer film has a nanostructured network of mass interwoven fibers with a diameter of 10–50nm. The copolymer obtained at the optimum conditions exhibits excellent redox activity in a 0.30M Na2SO4 aqueous solution (pH 11.0), which is superior to that of polyaniline itself. The improvement can be attributed to the synergistic effect of the COOH and OH functional groups in the copolymer chain.