We studied stationary kinetics of ascorbic acid oxidation in the presence of indole-3-acetic acid catalyzed by horseradish peroxidase. The catalytic (kcat and Km) and inhibition (Ki) constants were determined for pH from 4.5 to 7.0. The auxin proved to competitively inhibit the enzyme when a single ascorbic acid molecule is bound, while a non-competitive inhibition by IAA is observed for peroxidase oxidation of two or more substrate molecules. A mechanism of ascorbic acid oxidation in the presence of indole-3-acetic acid is proposed.