Sulfonated polyanilines (SPANs) with sulfur-to-nitrogen ratios of 0.05-0.40 were prepared by the copolymerization of aniline (ANI) and 3-aminobenzenesulfonic acid [metanilic acid (MA)] with ammonium peroxodisulfate in 1 M HCl at various MA/ANI and oxidant/monomer ratios and temperatures. The SPANs degree of oxidation, as ascertained from ultraviolet-visible and NMR spectra, and the SPAN bulk conductivity are inversely proportional to the content of MA units in the copolymer; that is, highly sulfonated SPAN samples are in a state closer to leucoemeraldine bases. The electron-withdrawing and steric effects of sulfonic groups clearly have a great impact on the structural features of SPANs and, consequently, on their properties, It has been suggested that the low conductivity of SPAN in comparison with polyaniline may be related to the low content of quinoid units in the SPAN molecules.