Photoreactions induced by ultraviolet radiation are among the most important external hazards for the integrity of DNA.1 The photolesion with the highest abundance is the [2 + 2] photoaddition of thymine bases adjacent on a DNA strand.2 This photoaddition yields a cyclobutane pyrimidine dimer (CPD). The formation of the CPD lesion has first been described in 1960.3,4 Nevertheless, the kinetics of its formation is still under debate. In solution the photodimerization of single thymine bases involves a triplet state.5,6 A reaction via a singlet channel is precluded in this diffusion limited process since singlet excitations in thymine are too short-lived.7,8 In DNA strands thymine bases are kept in proximity by the sugar-phosphate backbone and the diffusion limit does not apply. So in DNA the CPD lesion could in principle be formed via excited singlet or triplet states.