Described here are mechanistic details of the chemical reactivities of two modified/saturated pyrimidine residues that represent naturally occurring forms of DNA damage: 5-thyminy1-5,6-dihydrothymine, commonly referred to as the "spore photoproduct" (SP), and 5,6-dihydro-2'-deoxyuridine (dHdU), formed via ionizing radiation damage to cytosine under anoxic conditions and also serving as a general model of saturated pyrimidine residues. It is shown that due to the loss of the pyrimidine C5-C6 double bond and consequent loss of ring aromaticity, the C4 position of both these saturated pyrimidines is prone to the formation of a hemiaminal intermediate via water addition. Water addition is facilitated by basic conditions; however, it also occurs at physiological pH at a slower rate. The hemiaminal species so-formed subsequently converts to a ring-opened hydrolysis product through cleavage of the pyrimidine N3-C4 bond. Further decomposition of this ring-opened product above physiological pH leads to DNA strand break formation. Taken together, these results suggest that once the aromaticity of a pyrimidine residue is lost, the C4 position becomes a "hot spot" for the formation of a tetrahedral intermediate, the decay of which triggers a cascade of elimination reactions that can under certain conditions convert a simple nucleobase modification into a DNA strand break.