We have performed a series of UVRR measurements using 266 nm as excitation wavelength and in operando conditions. Indeed, in our experiment, the aqueous solutions of D, U, and D-U hyd were irradiated with UV light (about 40 mW/cm -1 ), and the resonance Raman spectrum was simultaneously collected from the same scattering volume as a function of the irradiation time. Although both D and U were already investigated in the context of UVRR, the related photodegradation aspects were not addressed. Besides, while small effects are expected from purine bases, in nucleobases such as U, the formation of CPD in specific conditions is a general and established photochemical mechanism that has been widely explored in experiments and simulations. Interestingly, our results point to negligible effects observed in pure U aqueous solutions (Figure ). On the other hand, for D (Figure ) and especially D-U hyd (Figure ) aqueous solutions at 288 K, we observed a systematic decrease of the signal that we possibly relate to a photodegradation pathway. Samples were prepared from crystalline compounds dissolved in pure water, with concentrations around 3-6 •10 -4 M providing good scattering profiles. We performed a set of 24 measurements at 288 K, each obtained by collecting UVRR signal for 10' (minutes) of exposition time and found that D-U hyd showed a decrease in scattering intensities in practically all vibrational peaks (Figure ). We have focused on a few wavenumber intervals and fit the time traces with monoexponential decay profiles of the kind I = I 0 exp (-bt) that reproduced the experimental trends reasonably well. It appears that U components in D-U hyd feature a slightly faster decay, and in general, the D-U hyd signal decays faster than for pure D (see Table for fit details). At 266 nm, the system showing the most intense scattering is U, probably due to the best match with resonance conditions. Possibly due to this reason, vibrations that could be attributed to U components also dominate in intensity for D-U hyd spectra (see Table for a list of attributions). Purine nucleobases, especially in the absence of sugar, have been generally considered quite resistant to UV light. Recently, Caldero-Rodriguez et al. have been investigating D photochemistry and photophysics, concluding that its behavior follows a fast non-radiative deactivation and/or a weak fluorescence essentially, while only 2% of D degrades under 287 nm light. Our results show that D is less resistant than expected. On the other hand, we expected U to manifest a photoreactive tendency, but we only observed this in the D-U assemblies. It is also important to stress that there is no consensus about the results of the photodimerization reactions obtained by many groups through the years. It appears that even when considering the same environment (e.g. fluid aqueous solutions), there are disagreements on the yields and the proportions of the different photoproductssuggesting these processes are all strongly dependent on a set of experimental conditions.