The velocity of photolysis of FAD was compared with those of FMN and riboflavin. The solutions of the substances were saturated with nitrogen or carbon dioxide and exposed to sunlight. The velocity of photolysis was FMN>riboflavin>FAD, and was higher in nitrogen than in carbon dioxide. An unknown substance having a Rf 0.08 (butanol・acetic acid・water) was separated by paper chromatography in the photolytic products. This compound contained phosphoric acid, ribose, adenine and is supposed to be deutero-derivative of FAD.