A study of the kinetics of the enzymically-catalyzed decarboxylation and oxidation of indoleacetic acid has provided evidence that it is a chain autoxidation initiated and propagated by two enzyme-controlled peroxidations. The following reaction sequence occurs:[Formula: see text]where S—COOH = indoleacetic acid; S∙ = skatole radical; SO2∙ = oxidized skatole radical or indole peroxide; SO2H = final reaction product; ROH = phenolic cofactor, i.e., resorcinol; RO∙ = semiquinol or aryl radical.