The UV electronic transition energies and their oscillator strengths for two stacked dimers having B-DNA geometries and consisting of 5-bz omouracil (U-Br) and a purine base were studied at the MS-CASPT2/6-311G(d) level with an active space of 12 orbitals and 12 electrons. The calculated energy of the first vertical (pi,pi*) transitions for the studied dimers remain in fair agreement with the maxima in the difference spectra measured for duplexes with the 5'-A(Br)U-3' or 5'-G(Br)U-3' sequences. Our MS-CASPT2 results show that the charge transfer (CT) states in which an electron is transferred from A/G to U-Br are located at much higher energies than the first (pi,pi*) transitions, which involve local excitation (LE) of U-Br. Moreover, CT transitions are characterized by small oscillator strengths, which implies that they could not be excited directly. The results of the current studies suggest that the :Formation of the reactive uracil-5-yl radical in DNA is preceded by the formation of the highly oxidative LE state of U-Br, which is followed by electron transfer, presumably from guanine.