Vibrational spectra of the lowest energy triplet states of thymine and its 2'-deoxyribonucleoside, thymidine, are reported for the first time. Time-resolved infrared (TRIR) difference spectra were recorded over seven decades of time from 300 fs -3 µs using femtosecond and nanosecond pumpprobe techniques. The carbonyl stretch bands in the triplet state are seen at 1603 and ~1700 cm −1 in room-temperature acetonitrile-d 3 solution. These bands and additional ones observed between 1300 and 1450 cm −1 are quenched by dissolved oxygen on a nanosecond time scale. Density-functional calculations accurately predict the difference spectrum between triplet and singlet IR absorption cross sections, confirming the peak assignments and elucidating the nature of the vibrational modes. In the triplet state, the C4=O carbonyl exhibits substantial single-bond character, explaining the large (~70 cm −1 ) red shift in this vibration, relative to the singlet ground state. Femtosecond TRIR measurements unambiguously demonstrate that the triplet state is fully formed within the first 10 ps after excitation, ruling out a relaxed 1 nπ* state as the triplet precursor.