The photooxidation of reduced diphosphopyridine nucleotide to diphosphopyridine nucleotide in the presence of various flavin compounds and methylene blue was demonstrated. In this photooxidative reaction, the potocatalytic activity was most intense in riboflavin, somewhat less intense in flavinmononucleotide and still less in flavinadeninedinucleotide. A strong activity was also found in methylene blue.