Solar UV light can be dangerous to cellular systems. One manifestation of this is the formation of thymine dimers, containing cyclobutane rings, in suitable regions of duplex DNA. Fortunately, these dangerous defects are rapidly recognized by a number of DNA repair enzymes, including the photolyase enzymes, whose activity is also triggered by the sun’s rays. This is thought to result in rapid electron transfer from the excited chromophores therein, to the pyrimidine dimer, resulting in opening of the cyclobutane ring. The electron then moves back, reconstituting the enzyme and the two pyrimidine monomer units, In an attempt to mimic this process in the absence of the enzyme, we have added radiation-generated electrons to thymine and uracil dimers at 77 K and used EPR spectroscopy to study the initial products. The results show that for trans-syn-1,3-dimethyluracil dimer, a long-lived dimer radical anion is formed, the electron being confined to one of the uracil units. On annealing, this was converted irreversibly into the monomer radical anion. In contrast, the cis-syn thymine dimer, and its structural analogue, the cis-syn-1,3-dimethyluracil derivative, gave the monomer anions directly at 77 K. These results confirm that electron addition is facile and that the dimer anions are very unstable. The results also show that both bonds break rapidly, rather than just one. We have also studied the structurally related dihydro derivatives in order to extend and confirm these conclusions.