Aniline was polymerized using horseradish peroxidase in a mixture of phosphate buffer and organic solvent. The structure of polyaniline was characterized by a FTIR spectrum. Enzymically synthesized polyaniline was composed of ortho-directed units and para-directed units, and the ratio of ortho-directed units to para-directed units was estimated from the absorption at the oop region of the FTIR spectrum. The structure of polyaniline depended on the pH of the phosphate buffer and the type of organic solvent. Polyaniline in which ortho-directed units were predominant was obtained by increasing the pH of phosphate buffer or by adopting an organic solvent with a high dielectric constant or log P. The reaction mechanism in the aqueous and organic solvent phase is proposed. The thermal and electric properties of ortho-directed polyaniline and para-directed polyaniline were compared by DSC and UV-Vis spectra. Ortho-directed polyaniline is estimated to be more thermally flexible, electrically conductive and soluble in N methylpyrrolidone solution.