A mediatorless H2O2 sensor based on coelectropolymerization of horse radish peroxidase (HRP) and o-phenylenediamine (o-PD) is described. The electrode responds to H2O2 in a few seconds and gives a current density of 73.3 nA 1 mu mol(-1) cm(-2) at -100 mV (versus AgAgCl). A linear calibration curve is obtained over the range 1-130 mu mol l(-1). From kinetic analyses, the response of the biosensor is rate limited by the enzyme kinetics, and the apparent Michaelis-Menten constant K’(m) is 220 mu mol l(-1). The electrode consisting of HRP entrapped in poly(o-phenylenediamine) film shows good stability because there is some interaction between HRP and o-PD in the electropolymerization.