A mechanistic model of peroxidase-catalyzed oxidation of indole-3-acetic acid (IAA) at neutral pH has been developed, characterized, and compared with experiments. The model is based on experimental facts showing that IAA is oxidized in the presence of HRP by two pathways : (i) the standard peroxidase cycle, which is accompanied by (ii) a nonenzymatic free radical chain reaction, The peroxidase cycle normally requires the addition of a hydroperoxide, whereas IAA oxidation does not. Therefore, the model includes the enzymatic peroxidase cycle which is initiated by organic hydroperoxide (ROOH) derived from autoxidation of IAA. It also includes a nonenzymatic free radical chain which utilizes oxygen, oxidizes IAA, and recycles ROOH required for the enzymatic cycle. Available experimental values of rate constants were used, Unavailable rate constants were initially estimated analytically using the steady state assumption and then optimized by computer simulation. There is a unique set of rate constants which satisfies the model, The average deviation of simulated kinetic traces from experimental ones was less than 5%. Critical values of the rate constants were determined; for values smaller than the critical values IAA oxidation stops. In the pre-steady state the concentration of ROOH rises exponentially, Thus, a decrease of the initial concentration of ROOH by 3 orders of magnitude gives rise to only a 1 min delay of reaction. For all intermediates except ROOH the steady state was reached in 1 min; for ROOH, 10 min after reaction initiation. More than half of the oxidation of IAA occurs by the chain reaction, a nonenzymatic pathway. The model satisfactorily describes all available experimental kinetic data and predicts some previously unobserved behavior which should stimulate further experiments.