The chemical polymerization of aniline and sodium diphenylamine-4-sulfonate produces poly(aniline-co-N-(4-sulfophenyl)aniline) (PAPSA), copolymers which have high molecular weights and are recovered as a dark green powder. The PAPSA copolymers have a monomer composition like the composition in the reaction mixture. They have conductivities which range between the conductivities of the poly(N-(4-sulfophenyl)aniline) homopolymer (0.0035 S/cm) and polyaniline (5.2 S/cm). While the homopolymer poly(N-(4-sulfophenyl)aniline) is 103 times less conductive than polyaniline, it is still 10(6) times more conductive than other polyaniline polymers with sulfonate groups. The conductivity and the ESR signal both decrease with the phenylsulfonic acid content in the copolymer. The PAPSA copolymers are soluble in aqueous NH4OH but not in aqueous HCl solutions. The color of the resulting solutions varies with the copolymer monomer composition. Films of the PAPSA copolymers on an electrode show two reversible redox reactions for the aniline units and one for the pendant phenylsulfonic acid groups between 0.2 and 0.8 V vs Ag/AgCl reference electrode, when immersed in aqueous acid solutions. During the redox process, the PAPSA films exhibit reversible color changes from pale yellow to green to dark blue.