Ab initio computations (CASSCF/6-31G* supported by CAS-PT2 single-point calculations) are used to study the reactivity of the triplet excited state of the guanine-cytosine DNA base pair. When the triplet excitation is centered on cytosine there is a competition between benign deactivation to the ground state and a hydrogen transfer route that can trigger double tautomerization. The calculated barriers favor the benign deactivation, but this route goes through a singlet/triplet intersystem crossing with small spin-orbit coupling. Therefore, the potentially mutagenic, double tautomerization route cannot be ruled out completely, and the two paths are probably an alternative to the well-known cytidine photodimerization reaction.