The bioelectrocatalytic function of two plant peroxidases, horseradish peroxidase (HRP) and a newly purified royal palm tree-leaf peroxidase (RPTP), was studied on graphite electrodes in aqueous buffer solutions, over the pH range 7.0 - 3.0, and in aqueous buffer solutions, at pH 6.0, containing different amounts of a polar organic cosolvent, i.e., ethanol and acetonitrile. The kinetics of direct and of mediated (using catechol as mediator) reduction of H2O2 at the HRP- and RPTP-modified graphite electrodes were analyzed amperometrically at - 50 mV(vs. Ag broken vertical bar AgCl broken vertical bar 0.1 M KCl) in a flow-through wall-jet electrochemical system. Values of the apparent rate constant of the heterogeneous electron transfer between the enzyme and graphite, k(s) the rate constant for enzymatic reduction of H2O2, k(1), and the rate constant of mediated reduction of H2O2, k(3), were determined using the modified Koutecky-Levich approach. Analysis of the variation of the rate constants and the response of the peroxidase-modified electrodes to H2O2 with pH and content of the organic cosolvent demonstrated that maximal peroxidase bioelectrocatalytic activity occurred at pH 5.0 - 6.0 and at concentrations of ethanol of 10 - 20% v/v. At a lower pH, higher concentrations of ethanol, and in aqueous solutions of acetonitrile, the bioelectrocatalytic activity of the peroxidase-modified electrodes drastically decreased. However, contrary to the data for homogeneous catalysis, both peroxidases were bioelectrocatalytically active even in 95% organic co-solvents, thus demonstrating a stabilizing effect of the enzyme immobilization on the bioelectrocatalytic performance of the peroxidases. RPTP immobilized on graphite demonstrated lower overall activity but a higher pH- and organic cosolvent-stability than HRP.