Enzymatic synthesis of a water-soluble, conducting polyaniline (PANT) was studied, using horseradish peroxidase as the biocatalyst and H2O2 as the initiator, in the presence of a poly(vinylsulfonic acid, sodium salt) (PVS) polyanion template. The effects of the buffer, concentration of H2O2, and the molar ratio of aniline to PVS on the polymerization were particularly investigated. The products were characterized by UV-vis/near-IR and FTIR spectroscopy, thermogravimetric analysis, and four-point probe conductivity measurement. The results showed that PVS could be chosen as a new template in the synthesis of PANI. The proper conditions of polymerization were obtained as follows: pH of the buffer was pH 4.0-5.0, the concentration of H2O2 was around 20 mM, and the molar ratio of PVS to aniline was 1-1.5. (c) 2005 Wiley Periodicals, Inc.