The photophysical and photochemical processes driving the formation of the ultraviolet (UV)-induced DNA Dewar lesion from the T(6-4)T dimer are investigated by time-resolved spectroscopy and quantum chemical modelling. Time-resolved absorption and emission spectroscopy in the UV revealed a biexponential decay of the electronically excited state (S1) with time constants in the 100 ps and 1 ns range. From the S1 state the system forms the Dewar lesion (proven by time-resolved infrared spectroscopy), the triplet state of the T(6-4)T dimer and the ground state of the original T(6-4)T dimer. The decay process from the excited singlet is activated and thus temperature dependent. Quantum chemical modelling is used to describe the reaction path via a minimum on the excited electronic potential energy surface in close proximity to a triplet state. The transition to the Dewar isomer competes with internal conversion and with triplet formation. Only if the backbone between the two thymines is closed, is the Dewar isomer formed with a significant yield. The simulations reveal that the tension built up by the backbone is required for guiding the reaction to the conical intersection leading to the Dewar isomer.