In pyrimidines with an unsubstituted C5 = C6 bond, H-addition via an anionic stage, the “ionization path”, occurs preferentially at position C6 and direct hydrogen addition, the “excitation path”, at position C5. In adenine derivatives, the “excitation path” yields C8-addition radicals and the “ionization path” C2-addition radicals. However, when the molecule is doubly protonated, C8-addition radicals are preferentially formed by protonation of the electron adduct. In pyrimidine crystals, the C5-addition radicals transform into C6-addition radicals upon irradiation with light of λ 400 nm. In purine crystals, the C8-addition radicals can also be transformed into C2-addition radicals in this way. In “van der Waals crystals”, this transformation is reversible upon storage at room temperature. In “polar crystals”, the transformation is irreversible. This indicates that the C6-addition radicals of pyrimidines and the C2-addition radicals of adenine derivatives need a polar environment to be stabilized. INDO calculations support this conclusion. While the “ionization path” could not be detected in “van der Waals crystals” both paths have been observed in “polar crystals”. These observations bear out the view that, in nonpolar environments, ionization is followed by geminate ion recombination with eventual subsequent homolytic dissociation of atomic hydrogen.