Molecular beacons (MB) are becoming more common as sequence-selective detectors of nucleic acids. Although they can easily detect single-base mismatches, they have never been used to directly detect DNA or RNA damage. To measure the degree of ultraviolet (UV) light damage in oligonucleotides, we report a novel MB approach for general detection of photoproducts in UV-irradiated rU₁₇ and dT₁₇ oligonucleotides. With monochromatic UV light irradiation at ca 280 nm under anoxic conditions, the oligonucleotide absorption decays with a single-exponential time constant of 123 ± 1 min for rU₁₇ and with double-exponential time constants of 78 ± 0.5 min (99%) and 180 ± 5 min (0.05%) for dT₁₇ oligonucleotides. Under the same conditions, the MB fluorescence decays more quickly, with single-exponential time constants of 19 ± 2 and 26 ± 3 min for rU₁₇ and dT₁₇, respectively. Similar kinetics were observed with broadband UV light irradiation of oligonucleotides. The differences in the UV damage kinetics of dT₁₇ and rU₁₇ and their detection by absorption and fluorescence techniques will be discussed in the context of differential instabilities introduced in the nucleic acid–MB duplex by the different photoproducts formed.