Oxidatively generated damage to DNA induced by a pyrenyl photosensitizer residue (Py) covalently attached to a guanine base in the DNA sequence context 5′-d(CAT[G1Py]CG2TCCTAC) in aerated solutions was monitored from the initial one-electron transfer, or hole injection step, to the formation of chemical end-products monitored by HPLC, mass spectrometry, and high-resolution gel electrophoresis. Hole injection into the DNA was initiated by two-photon excitation of the Py residue with 355 nm laser pulses, thus producing the radical cation Py•+ and hydrated electrons; the latter are trapped by O2, thus forming the superoxide anion O2•+. The decay of the Py •+ radical is correlated with the appearance of the G •+/G(-H)• radical on microsecond time scales, and O2•- combines with guanine radicals at G 1 to form alkali-labile 2,5-diamino-4H-imidazolone lesions (Iz 1Py). Product formation in the modified strand is smaller by a factor of 2.4 in double-stranded than in single-stranded DNA. In double-stranded DNA, hot piperidine-mediated cleavage at G2 occurs only after G1Py, an efficient hole trap, is oxidized thus generating tandem lesions. An upper limit of hole hopping rates, khh < 5 × 103 s-1 from G1 •+-Py to G2 can be estimated from the known rates of the combination reaction of the G(-H)• and O2 •- radicals. The formation of Iz products in the unmodified complementary strand compared to the modified strand in the duplex is ∼10 times smaller. The formation of tandem lesions is observed even at low levels of irradiation corresponding to "single-hit" conditions when less than ∼10% of the oligonucleotide strands are damaged. A plausible mechanism for this observation is discussed.