Potosensitized formation of 8-hydroxyguanine in DNA by riboflavin was observed. A reaction mechanism involving guanine radical cation and hydration reaction was proposed. This hypothesis was confirmed by the incorporation of [18O]-atom within guanine moiety in isotopic experiments using [18O]-H2O. Photosensitized formation of oh8Gua by riboflavin was also observed in cellular DNA.