Although the peroxidase-oxidase (PO) oscillator has been widely studied [Scheeline et a]. Chem. Rev. 1997, 97, 739],(1) the continued lack of quantitative agreement between experiment and theory suggests that the molecular components and reactions essential for oscillation dynamics are not fully understood. Particularly, the role of photochemical reactions has been largely overlooked even though the photosensitizer methylene blue (MB+) has been routinely added to PO reaction mixes by investigators since the first demonstration of sustained oscillations [Nakamura et al. Nature 1969, 222, 794].(2) We reasoned that the presence of MB+ in the PO reaction should make oscillations sensitive to visible light exposure. We tested this possibility and observed that both the frequency and the amplitude Of O-2 oscillations obtained with a free-running periodic PO oscillator were suppressed in a rapid and reversible manner when exposed to visible light. The effect occurred at illumination wavelengths between 600 and 700 nm, was greatest at 670 nm, and was consistent with the absorbance spectrum of MB+. Measurements of the rate of NADH oxidation by MB+ during illumination with red light showed a dose dependence consistent with the response curve observed for periodic PO oscillations. We concluded that PO oscillations were influenced by light through a photochemical effect on MB+, with the photosensitive reaction being the oxidation of NADH by MB+. Given these results, photoinduced suppression effects may be a common experimental bias in PO experiments where broadband spectroscopic illumination has been used. Future laboratory work involving the PO oscillator will need to control for MB+-dependent photochemical effects and theoretical modeling efforts should account for photochemistry involving MB+ and NADH.