The structure and doping behaviors of poly(p-styrene sulfonic acid) (PSSA)-doped polyaniline(PANi) and poly( 2-acrylamido-2-methyl-1-propane sulfonic acid) (PAMPS)-doped PANi were investigated by conductivity measurement, ultraviolet-visible spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. The conductivities of the PSSA-doped PANi composites are higher by up to two orders of magnitude than those of the PAMPS-doped PANi composites of the same composition, given by acid-to-aniline ratio. The confinement of pendant benzene sulfonic acid groups in the PSSA and that of pendant amino methyl propane sulfonic acid groups in the PAMPS may result in non-uniform and inefficient doping, such that some of the acid groups are not able to participate in the doping. The conductivity difference between the two different polymeric acid-doped PANi composites can be explained by the difference in the degree of the steric hindrance between the two polymeric acid dopants. The PSSA-doped and PAMPS-doped PANi systems reveal the same optimum composition of 1/1 (moles of sulfonic acid units in the polymeric acids/moles of aniline units in PANi) for maximum conductivity. The maximum conductivity seems to be attributed to the increase in the doping level and the decrease in the order and packing density of the PANi subchain alignment with increase of the acid-to-aniline ratio. (C) 1998 Elsevier Science S.A.