Nucleobase-derived radicals are crucial transient intermediates in the radiation damage of nucleic acids (RNA and DNA). Among the recognized mechanisms of radiation damage, capture of a low-energy electron followed by protonation of the transient anion radical results in the formation of hydrogen-atom adducts that can undergo further degradation reactions. [1] Radiation damage has been studied extensively by pulse radiolysis in solution and the solid state, in which characterization of the transient nucleobase radicals relied mainly on the detection of species with unpaired electrons by electron paramagnetic resonance (EPR) spectroscopy. In particular, cytosine and cytosine-containing nucleosides have been the subject of numerous studies that reported the formation of adducts with the hydrogen atom positioned at C5, C6, N3, and N7 in the solid state (Scheme 1). [2] However, the identity of these radicals has been subject to discussion, and the factors that affect their formation and kinetic and thermodynamic stability have not been established unequivocally by EPR in the complex mixtures produced by pulse radiolysis. EPR analysis can be further complicated by the formation of cation radicals upon Scheme 1. Formation of cytosine radicals by the direct mechanism.