Mass spectrometric and ultraviolet absorption spectral evidence are presented for the assignment of structures to three products detected in the reaction mixtures formed upon the photolysis of aqueous solutions of the nucleotide 2′-deoxyguanosine 5′-monophosphate (dGMP) with light of wavelengths >270 nm. The products for which structures are assigned are spiroiminodihydantoin 2′-deoxyribonucleotide (1), 2,2-diamino-4-([2-deoxy-5-monophosphate-β-d-erythro-pentofuranosyl]amino)-5-(2H)-oxazolone (oxazolone 2′-deoxyribonucleotide, 2) and 2-amino-5-([2-deoxy-5-monophosphate-β-d-erythro-pentofuranosyl]amino)-4H-imidazol-4-one (imidazolone 2′-deoxyribonucleotide, 3). These results, when combined with mechanistic data presented in an earlier communication, provide support for the proposal that the irradiation of dGMP with UVB light leads to the formation of singlet oxygen. The UV absorption spectral properties of the imidazolone make this product a reasonable candidate to rationalize the autosensitization of dGMP degradation reported in the earlier communication.