The peroxidase-NADH oscillator examined here initially consists of four chemical components. The well-mixed aqueous solution includes native horseradish peroxidase, reduced beta-nicotinamide adenine dinucleotide (NADH), methylene blue (MB(+)), and dissolved oxygen combined in a semi-batch reactor under a set of standard conditions. In this system, the macroscopic appearance of the process of oxygen dissolution from the gas phase is dependent on k(m), the mass transport constant of oxygen out of solution. Additional details of oxygen mass transport are derived. The amplitude of oxygen oscillations is decreased by continuous illumination by the deuterium source of a diode array spectrophotometer. This attenuation effect of light is dependent on wavelengths less than or equal to 248 nm. Initial omission of MB(+) allows several damped oscillations of small amplitude. Subsequent addition of MB(+) to the oscillator results in oscillations of much larger amplitude. MB(+) is seen to either directly or indirectly enhance the conversion of peroxidase compound ill to the native enzyme and then inhibit oxygen consumption, allowing the initiation of relatively large, prolonged oscillations. MB(+) is seen to function either as a system catalyst, or as a peroxidase inhibitor in the oxidation of NADH by oxygen.