A copolymer, poly(aniline-co-o-aminophenol), was prepared chemically by using ammonium peroxydisulfate as an oxidant. The monomer concentration ratio of o-aminophenol to aniline strongly influences the copolymerization rate and properties of the copolymer. The optimum composition of a mixture for the chemical copolymerization consisted of 0.3 M aniline, 0.021 M o-aminophenol, 0.42 M ammonium peroxydisulfate, and 2 M H2SO4. The result of cyclic voltammograms in a potential region of -0.20 to 0.80 V (vs.SCE) indicates that the electrochemical activity of the copolymer prepared under the optimum condition is similar to that of polyaniline in more acid solutions. However, the copolymer still holds the good electrochemical activity until pH 11.0. Therefore, the pH dependence of the electrochemical property of the copolymer is improved, compared with poly(aniline-co-o-aminophenol) prepared electrochemically, and is much better than that of polyaniline. The spectra of IR and H-1 NMR confirm that o-aminophenol units are included in the copolymer chain, which play a key role in extending the usable pH region of the copolymer. The visible spectra of the copolymers show that a high concentration ratio of o-aminophenol to aniline in a mixture inhibits the chain growth. (c) 2007 Wiley Periodicals, Inc.