ion complexes containing horseradish peroxidase (HRP) were prepared on the electrode surfaces from a sulfonated polyaniline, poly(2-methoxyaniline-5-sulfonic acid) (PMAS) and a polycation, poly((L)-lysine) (PLL) or poly(ethyleneimine) (PEI). The detection limit of the PMAS-PLL-HRP electrode for H2O2 is 10(-8)-10(-7) M at pH 4.5-8.4. The positive potential limit for the reductive H2O2 sensing varies from 0.0 to + 0.5 V vs. Ag \ AgCl as the pH decreases from 8.4 to 4.5. The catalytic activity of HRP, and the electrochemical activity or electronic conductivity of PMAS are essential to the response. The sensitivity of this sensor is comparable to that of an HRP electrode sith electropolymerized polypyrrole, although the present sensor is much easier to prepare, just by casting a mixed solution of an enzyme and polymers, without electrochemical polymerization.