The effect of pH on the kinetics of the elect to reduction of H2O2 catalysed by horseradish peroxidase (HRP) has been studied with LSV in the potential range from 700 to -50 mV versus SCE (under steady-state conditions and with an RDE system) and at -50 mV versus Ag/AgCl on HRP-modified graphite electrodes placed in a wall-jet flow-through electrochemical cell. Increasing [H3O+] was shown to enhance significantly the current of the bioelectro reduction of H2O2 due to direct electron transfer (ET) between graphite and the enzyme over the potential range involved. It is demonstrated that at high overvoltages (E < 0.2 V) H3O+ does not affect the rate of the enzymatic reduction of H2O2, but it increases the rate of direct ET between graphite and HRP. The values of the apparent rate constant of heterogeneous ET between HRP and graphite, k(s), changed from a value of 0.54 +/- 0.05 s(-1) in phosphate buffer solution (PBS) at pH 7.9, to a value of 11.0 +/- 1.7 s(-1) in PBS at pH 6.0. Analysing the pH rate profile and the variation of the k(s) with increasing [H3O+] made it possible to consider the reaction mechanism as implying the participation of a proton in the limiting step of charge transfer.