1. The peroxidase (donor: H2O2 oxidoreductase, EC 1.11.1.7) catalyzed oxidation of reduced nicotinamide adenine dinucleotides, dihydroxyfumaric acid or indole-3-acetic acid, in a reaction system open to O2, exhibit damped oscillations in the reaction rate. 2. All known aerobic oxidation reactions catalyzed by peroxidase alone are autocatalytic. The damped oscillation is a consequence of the autocatalysis. 3. Compound III is observed spectroscopically in the NADH and the dihydroxyfumaric acid oxidation but not in the indole-3-acetic acid oxidation. Large differences exist in the Compound III kinetics in the NADH and the dihydroxyfumaric acid oxidation. Compound III is not essential in the mechanism of the oscillation. 4. The oxidation of the three donors is accompanied by chemiluminescence whose intensity increases in the order NADH < dihydroxyfumaric acid < indole-3-acetic acid. Except in the NADH oxidation the chemiluminescence is strong enough to be used for kinetic measurements and possibly also for spectroscopic identification of intermediates. © 1969.