Poly(aniline-co-2-amino-4-hydroxybenzenesulfonic acid) (PAAHB) was first synthesized via the electrochemical opolymerization of aniline and 2-amino-4-hydroxybenzenesulfonic acid (AHB) in the presence of an ionic liquid. The conductivity of PAAHB in the oxidized state is 0.45 S cm(-1). The conductivity and ESR spectra of PAAHB are slightly affected by water. The cyclic voltammograms of PAAHB reveal excellent redox activity from PH < 1 to PH 11.0. This is attributed to the synergistic effect of the -SO3- and -OH functional groups in the copolymer chain and the ionic liquid incorporated into the PAAHB film. It is evident that the PH dependence of the redox activity and conductivity of PAAHB is much better than that of polyaniline. The proton NMR spectra of PAAHB and AHB; demonstrate that the -SO3- group exists in the copolymer chain instead of the -SO3H group. Therefore, PAAHB can be used for the determination of dopamine in the presence of ascorbic acid as a resulf of the -SO3- group, which plays an important role in the selectivity.