Studies of the kinetic behavior of horseradish peroxidase (HRP) at pH 8 and at room temperature indicate that the reaction of phenol with H2O2 catalyzed by HRP exhibits normal Michaelis-Menten saturation kinetics. An irreversible reaction mechanism for the steady-state kinetics of HRP, which is consistent with the experimental data, is considered. The second-order rate constants for the reactions of HRP with H2O2 and compound II with phenol are 4.14 x 10(5) M(-1)s(-1) and 5.54 x 10(4) M(-1)s(-1), respectively.