Hydrogen peroxide, the oxidant substrate of peroxidase, is also an inactivating agent of this enzyme. The reductant substrates protect the enzyme from the inactivating process. A reaction mechanism is proposed, in which two competitive routes exist for Compound I of peroxidase; one catalytic and one inactivating. The analytical solution produced at the end of the reaction supports the proposed mechanism and shows the dependence between the number of turnovers of the enzyme (r) and the ratio of both substrates.