The higher formation yields measured in the ultrafast photoinduced formation of cyclobutane thymine dimers (T < > T) with respect to those of cytosine (C < > C) are explained, on the basis of ab initio CASPT2 results, by the existence in thymine of more reactive orientations and a less efficient photoreversibility, whereas in cytosine the funnel toward the photolesion becomes competitive with that mediating the internal conversion of the excited-cytosine monomer.