The study addresses interconnected issues related to two major types of cycloadditions between adjacent thymines in DNA leading to cyclobutane dimers (T <> Ts) and (6-4) adducts. Experimental results are obtained for the single strand (dT)(20) by steady-state and time-resolved optical spectroscopy, as well as by HPLC coupled to mass spectrometry. Calculations are carried out for the dinucleoside monophosphate in water using the TD-M052X method and including the polarizable continuum model; the reliability of TD-M052X is checked against CASPT2 calculations regarding the behavior of two stacked thymines in the gas phase. It is shown that irradiation at the main absorption band leads to cyclobutane dimers (T <> Ts) and (6-4) adducts via different electronic excited states. T <> Ts are formed via (1)pi pi* excitons; [2+2] dimerization proceeds along a barrierless path, in line with the constant quantum yield (0.05) with the irradiation wavelength, the contribution of the (3)pi pi* state to this reaction being less than 10%. The formation of oxetane, the reaction intermediate leading to (6-4) adducts, occurs via charge transfer excited states involving two stacked thymines, whose fingerprint is detected in the fluorescence spectra; it involves an energy barrier explaining the important decrease in the quantum yield of (6-4) adducts with the irradiation wavelength.