The 2'-deoxyadenosine center dot center dot center dot 2'-deoxythymidine (dAdT(center dot-)) radical anion nucleoside pair has been investigated both experimentally and theoretically in the gas phase. The vertical detachment energy (VDE) and adiabatic electron affinity (AEA) were determined by anion photoelectron spectroscopy (PES). The measured photoelectron spectrum features a broad band having an onset at similar to 1.1 eV and a maximum at the electron binding energy (EBE) ranging from 1.7 to 1.9 eV. Calculations performed at the M06-2X/6-31++G** level reveal that the observed PES signal is probably due to a dAdT(center dot-) complex in which the thymine of the dT nucleoside forms hydrogen bonds that engage its 07 and 08 atoms as well as the 3'- and 5'-hydroxyl groups of 2'-deoxyadenosine (dA), while dT's 3'-hydroxyl group interacts with the N1 of dA. In this heterodimer, the excess electron is entirely located on thymine. The biologically relevant Watson-Crick arrangement of the dAdr(center dot-) dimer was found to be substantially less stable (by similar to 19 kcal mol(-1) in Gibbs free energy scale) than the above-mentioned configuration; hence, it is not populated in the gas phase.