Copolymer nanoparticles from aniline (AN) and 4-sulfonic diphenylamine (SDP) were simply synthesized for the first time by an oxidative precipitation polymerization with inorganic oxidants in an acidic aqueous medium without any external emulsifier or stabilizer. The polymerization yield, intrinsic viscosity, solubility, solvatochromism, electrical conductivity, and thermal stability of the copolymers were systematically studied through changes in the AN/SDP ratio, polymerization temperature, oxidant species, monomer/oxidant ratio, and acidic medium. The molecular structure of the copolymers was characterized with elemental analysis, IR, and ultraviolet-visible spectra. The polymers exhibited very good solubility in polar solvents, water, and NH4OH, and this was mainly attributable to the presence of sulfonic acid side groups. The electrical conductivity of the copolymers increased greatly, from 6.00 x 10(-4) to 2.55 X 10(-1) S/cm, with increasing AN content. The size of the copolymer particles, determined by laser particle analysis and atomic force microscopy (AFM), strongly depended on the polymer state and oxidant/monomer ratio. Pure dedoped particles of the AN/SDP (50/50) copolymer at an oxidant/monomer ratio of 1/2 exhibited minimum length/diameter ratios of 62/44 and 45/30 nm by AFM and transmission electron microscopy, respectively. The copolymers showed typical four-step weight-loss behavior in nitrogen and air and higher thermostability in nitrogen. (C) 2004 Wiley Periodicals, Inc.