A multinuclear magnetic resonance and vibrational study on 5-bromouridine and its base pairing with guanosine and adenosine in deuterated dimethyl sulfoxide at different concentrations is reported. A dicarbonylic non-self-associated form is suggested for 5-bromouridine on the basis of the 1 H-, 15 N-NMR, and Raman data. When guanosine is added in equimolar amounts, a downfield shift of the (N 3 )H proton of 5bromouridine and of the (N 1 )H and NH 2 protons of guanosine is observed; these results can be interpreted, according to the Raman ones, considering that only a fraction of guanosine is ''wobble base paired'' with 5-bromouridine, whereas the remaining part is self-associated. When 5-bromouridine is mixed with adenosine, the proton chemical shift of the aminic NH 2 of adenosine increases and the (N 3 )H iminic of 5-bromouridine moves downfield at a value higher than that observed for the 5-bromouridine-guanosine mixture. This behavior supports the hypothesis that 5-bromouridine interacts more with adenosine than with guanosine, but the results obtained are not able to establish which type of pairing (Watson-Crick or Hoogsteen) is present. Finally, the infrared spectrum of the solid 5-bromouridine : adenosine adduct, for which X-ray measurements of other authors suggested a Hoogsteen pairing, is reported and the observed bands are discussed.