Urea residues are produced by ionizing radiation on thymine residues in DNA. We have studied an oligodeoxynucleotide containing a thymine opposite the urea residue, by one and two dimensional NMR spectroscopy. The urea deoxyribose exists as two isomers with respect to the orientation about the peptide bond. For the trans isomer we find that the thymine and urea site are positioned within the helix and are probably hydrogen bonded. The oligonucleotide adopts a globally B form structure although conformational changes are observed around the mismatch site. A minor species is observed, in which the urea deoxyribose and the opposite base adopt an extrahelical position and this corresponds to the isomer cis for the peptide bond.