Self-doped poly(aniline-co-aminonaphthalene sulfonic acid) (PANI-ANSA) was synthesized by the copolymerization of 5-aminonaphthalene-2-sulfonic acid (ANSA) and aniline. Scanning electron microscopy and transmission electron microscopy showed that the morphology of PANI-ANSA synthesized at a high molar ratio of aniline to ANSA was nanotubular, but at a low molar ratio, only a granular morphology formed. A possible formation mechanism for nanotubes was proposed. PANI-ANSA had better thermal stability than HCl-doped polyaniline; the highest onset decomposition temperature was as high as 340degreesC because of -SO3H linked with the polymer backbone by a covalent bond. PANI-ANSA was partially soluble in basic solutions, and its conductivity was between 10(-2) and 10(-4) S/cm, depending on the sulfonation degree.