The propensity of 5-formyluracil and 5-formylcytosine, i.e. oxidative lesions and epigenetic intermediates, in acting as intrinsic DNA photosensitizers is unraveled by using a combination of molecular modeling, simulation and spectroscopy. Exploration of potential energy surfaces and non-adiabatic dynamics confirm a higher intersystem crossing rate for 5-formyluracil, whereas the kinetic models evidence different equilibria in the excited states for both compounds.