We have recorded NOESY spectra of two non-selfcomplementary undecanucleotide duplexes. Frma the observed NOEs we do not detect any significant distortion of the helix when a 6-C pair is replaced by a G-T pair and the normal interresidue connectivities can be followed through the mismatch site. We conclude that the 2D spectra of the non-exchangeable protons do not allow differentiation between a wobble or rare tautmer form for the mismatch. NOE measurements in HIO, however, clearly show that the mismatch adopts a wobble structure and give information on the hydration in the minor groove for the G-T base pair which is embedded between two A-T base pairs in the sequence.