Possible radical reaction products observed when subjecting monohydrate crystals of the DNA base cytosine to ionizing radiation are characterized and analyzed by means of density functional theory. Comparison is made with data from a recently published detailed ESR and ENDOR study by Sagstuen et al. (Sagstuen, E.; Hole, E. O.; Nelson, W. H.; Close, D. M. J. Phys. Chem. 1992, 96, 8269), as well as earlier studies on methylcytosine and cytidine monophosphates. For cytosine monohydrate it is found, when comparing computed and measured radical hyperfine coupling constants, that products other than those initially assumed are possibly being formed. Instead of the original model that irradiation leads to the net reaction of dehydrogenation at the N1 position of one cytosine molecule and hydrogenation at the N3 position of a second cytosine, we present an alternative mechanism where water is involved in the process. This alternative mechanism leads to the formation of N3 hydrogenation and C5 hydroxylation net products, as the main reactions. Not only do the hyperfine couplings provide a better match for the latter but they are also energetically favored over the first mechanism.