Frozen aqueous solutions of thymine and its derivatives were irradiated with visible light (lambda greater than 320 nm) in the presence of various furocoumarins. ESR analysis revealed the induction of hydrogen adduct free radicals at C-6 position of thymine, only with those furocoumarin derivatives which show a skin-photosensitizing ability. It has been shown, moreover, that the photocycloaddition of psoralen to thymine, which is responsible for the biological effects of this dye, is inhibited when the induction of free radicals in thymine moiety has been prevented by electron scavengers. It is suggested that the free radicals observed could be involved in the biological photosensitization. The mechanism of free radical generation and singlet oxygen production by furoccoumarins were also investigated.