Abstract—The mechanism of the photooxidation of ascorbic acid by flavin mononucleotide has been studied by steady state and flash photolysis techniques. The primary reaction consists of an electron abstraction from ascorbic acid by the flavin singlet and triplet states by a diffusional process and in the latter case also by the formation of a triplet flavin‐ascorbic acid complex. Under anaerobic conditions, the secondary reactions consist primarily of a radical recombination and a ‘dark’chemical back reaction, leading to the reformation of unchanged flavin and ascorbic acid. In the presence of oxygen, the ‘back reactions’are prevented, resulting in the efficient photooxidation of as