The primary photoproducts of 5mCyd elute at ca. 3.7, 8.5, and 10.9 minutes and exhibit absorption bands with absorption below ca. 310 nm (Figure bottom, left). Similarly, the primary photoproducts of Cyd elute with a retention time of ca. 3.3, 5.0, and 5.5 minutes and exhibit absorption bands below ca. 300 nm (Figure bottom, right). The similar retention times and absorption spectra for the primary photoproducts of 5mCyd and Cyd suggest that their structures should be similar, but are formed in significantly less amount (i.e., ca. 3-fold) in 5mCyd than in Cyd. These observations further suggest that the excited state(s) responsible for their formation is(are) the same in both nucleosides but it is(are) populated in significantly less yield in 5mCyd than in Cyd, whether at pH of 5.6 or 7.4. The report of a longer-lived 1 n N * state in this Letter, and in previous works, which is either formed in significantly less yield and cannot be detected in 5mCyd (Figure ), lend indirect support to the idea that the 1 n N * excited state participates in the formation of one or more of these photoproducts. This idea, together with the fact that the stimulated emission occurs in the same spectral region, could explain why 1 n N * excited state in 5mCyd is not detected in Figures 2, S2 Figure compares the absorption spectrum of the Cyd photoproduct eluting at 3.3 minutes with the absorption spectrum of 6-hydroxycytidine photohydrate reported by Fisher and Johns 39 at pH 8.4. Both absorption spectra closely resemble each other. The exception is the high-energy absorption band around ca. 210 nm, which has somewhat lower intensity in the 6-hydroxycytidine photoproduct compared to the photoproduct eluting at 3.30 min. We have also optimized the ground state of 6-hydroxycytosine in a hybrid 5 explicit waters (5W) with implicit CPCM water solvation model and simulated its absorption spectrum at the TD-PBE0-D3BJ/CPCM(+5W)/def2-TZVPD//B3LYP G -D3BJ/CPCM(+5W)/def2-TZVPD level of theory. The simulated absorption spectrum compares fairly well with the absorption spectra of the 6hydroxycytidine photoproduct (and that eluting at 3.3 min). Once again, the exception is with the highenergy band around 210 nm that has significantly more intensity in the simulated spectrum for 6hydroxycytosine (Figure ). Taking into consideration that the absorption of the photoproduct eluting at 3.3 min was collected using a photodiode array detector of a HPLC instrument and at pH of 5.6, while that of the 6-hydroxycytidine photoproduct was recorded at pH of 8.4, presumably in a UV-vis spectrophotometer, we argue that the agreement between both spectra is quite satisfactory. Collectively, the experimental and computational data reported herein suggest that the photoproduct eluting at 3.3 min is likely the 6-hydroxycytidine photoproduct.