NMR studies have been performed on the association of 3',5'-di-O-acetyl-2'-deoxyuridine with a 2'-deoxyadenosine derivative in solution. Using a CDClF2/CDF3 solvent mixture, measurements at very low temperatures allowed the observation of individual complexes ih the slow-exchange regime. From 2D NOE connectivities of the imino proton resonances at 125 K the predominant species could be assigned to a Watson-Crick geometry. However, concentration-dependent N-15 chemical shifts of specifically labeled uridine and adenosine nucleosides indicate a considerable population of Hoogsteen base pairs at ambient temperatures. These results together with a more downfield chemical shift of the Watson-Crick bound imino proton point to a stronger hydrogen bond in Watson-Crick when compared to Hoogsteen base pairs.