Green plants grown in a closed, illuminated system liberate small quantities of carbon monoxide. Similarly, finely divided powder and chlorophyll extracts of green plants, when illuminated in an environment of oxygen and water, will yield small quantities of carbon monoxide as well as certain aldehydes. The component of the light spectrum which is absorbed in photosynthesis (480 to 680 mmicro) was found to be responsible for the CO and aldehyde phenomena.