A novel water-soluble polyaniline has been synthesized by horseradish peroxidase catalyzed oxidative free-radical coupling of 2,5-diaminobenzenesulfonate. The polymerization reaction is rapid, and the average molecular weight of the resulting polymer is on the order of 18 000. Unlike the sulfonated polyanilines synthesized through posttreatment of polyaniline with fuming sulfuric acid, polyaniline reported here is fully sulfonated. This electrochemically active polymer shows strong pH dependence of absorption and other physical characteristics. This class of novel electroactive polyelectrolytes can be self-assembled into multilayer structures by interleaving with a variety of polyelectrolytes.