HCl-doped emeraldine form of polyaniline (PANI-H) was synthesized by oxidative polymerization of aniline in aqueous hydrochloric acid solution using potassium dichromate as an oxidant. The undoped form of PANI-H (i.e. PANI) was chemically reduced using hydrazine as the reductant. The structure, doping, conductivity and thermal stability of the reduced polyaniline (PANI-R) were studied by elemental analysis, FT-i.r., solid-state C-13-NMR, XPS and TGA. It was found that most of the quinoid structural units in PANI were reduced to benzenoid structural units. PANI-R had been doped with HCl and iodine, but only the iodine-doped product (PANI-RI) showed high electrical conductivity. FT-i.r. and XPS results indicated that some of the benzenoid structural units in PANI-R were oxidized to quinoid structural units during the iodine-doping process and a highly conjugated pi system might be formed in the PANI-RI molecular chains. TGA results indicated that PANI-R had better thermal stability than PANI. The dopants-HCl and iodine-were almost completely removed from the HCl and iodine-doped polyanilines below 250 degrees C. PANI-R could also be oxidized by potassium dichromate. The thermal stability of the intrinsically oxidized PANI-R (PANI-RO) was unexpectedly poor. The electrical conductivity of HCl-doped PANI-RO was much lower than that of HCl-doped PANI.