Employing the electrochemical method, horseradish peroxidase (HRP) was immobilized on polyaniline films polymerized on the platinum foil and on glassy carbon to form two kinds of the enzyme electrode, which are designated as HRP/PAN/Pt and HRP/PANC, respectively. The HRP/PANPt electrode potential was set at 0.20 V (vs. SCE), and that of HRP/PANC was set at 0.075 V (vs. SCE). They have a fast response. The response currents of both enzyme electrodes in 0.2 M phosphate buffer containing 1 x 10(-5) M H2O2 increase with decreasing pH and potential. The apparent Michaelis-Menten constant kappa’(m) is 3.2 x 10(-5) M for the HRP/PANPt electrode, and is 2.8 x 10(-5) M for the HRP/PANC electrode. The activation energy of the enzyme-catalyzed reaction is 15.4 kJ mol(-1) for the HRP/PANPt electrode, and is 13.8 kJ mol(-1) for the HRP/PANC electrode. The electrochemical reduction of hydrogen peroxide was observed at both the bare platinum electrode and polyaniline film polymerized on platinum, but the current of the reduction of hydrogen peroxide at the polyaniline film is very small. Hydrogen peroxide cannot be reduced at either the bare glassy carbon or polyaniline film polymerized on the glassy carbon. Both enzyme electrodes have a linear relationship between the response current and concentration of hydrogen peroxide below 5 x 10(-6) M in the mediator-free solution and at low potentials, so the polyaniline horseradish peroxidase electrodes can be used to determine low H2O2 concentrations.