The study of the photosensitization mechanism of dGMP by flavin adenine dinucleotide (FAD) as sensitizer was carried out by time-resolved 308 and 248 nm laser flash photolysis. Direct evidence of electron transfer from dGMP to either triplet state of FAD ((3)FAD(*)) or oxidized FAD radical (FAD(.+)/FAD(-H)(.)) was obtained. A comparison of nucleotide-free and -bound riboflavin suggests that the nucleotide environment does not markedly influence sensitizer reactivity.