The water-soluble conducting copolymer of poly(aniline-co-o-aminobenzenesulfonic acid) (PAOABSA) was synthesized directly from aniline and o-aminobenzenesulfonic acid and characterized by elemental analysis, FTIR, W-Vis, X-ray powder diffraction, and XPS. The room-temperature conductivity of the copolymer can be changed by controlling the molar fraction of the starting materials, and its maximum conductivity at room temperature can reach 3.4 S/cm, which is comparable with polyaniline (PANI) synthesized by the standard method and sulfonated polyaniline (SPAN) having a high degree of sulfonation reported by Wei et al. it was found that the room-temperature conductivity of the copolymer decreases with increase of the degree of sulfonation because of the introduction of the -SO3- group reducing the conjugation of the polymer chain. On the other hand, its solubility in 1.0M NaOH solution increases with increase of the degree of sulfonation due to the -SO3- group linking directly on the phenyl rings of the polymer chain.