The photosensitized hole injection and guanine base damage phenomena have been investigated in the DNA sequence, 5′-d(CATG1PyCG2TCCTAC) with a site-specifically positioned pyrene-like (Py) benzo[a]pyrene 7,8-diol 9,10-epoxide-derived N2-guanine adduct (G1Py). Generation of the Py radical cation and subsequent hole injection into the DNA strand by a 355 nm nanosecond laser pulses (∼4 mJ cm−2) results in the transformation of G1Py to the imidazolone derivative Iz1Py and a novel G1Py* photoproduct that has a mass larger by 16 Da (M+16) than the mass (M) of G1Py. In addition, hole transfer and the irreversible oxidation of G2, followed by the formation of Iz2 was observed (Yun et al. , J. Am. Chem. Soc., 129, 9321). Oxygen-18 and deuterium isotope labeling methods, in combination with an extensive analysis of the MS/MS fragmentation patterns of the individual dGPy* nucleoside adduct and other data show that dGPy* has an unusual structure with a ruptured cyclohexenyl ring with a carbonyl group at the rupture site and intact guanine and pyrenyl residues. The formation of this product competes with hole injection and thus diminishes the efficiency of oxidation of guanines within the oligonucleotide strand by at least 15% in comparison with that in the dGPy nucleoside adduct.