The structure of the hexitol nucleic acid (HNA) h(GCGCTTTTGCGC) was determined by NMR spectroscopy. This unnatural nucleic acid was developed as a mimic for A-RNA. In solution, the studied sequence is forming a symmetric double-stranded structure with four central consecutive T.T wobble pairs flanked by G.C Watson-Crick base pairs. The stem regions adopt an A-type helical structure. Discrete changes in backbone angles are altering the course of the helix axis in the internal loop region. Two H-bonds are formed in each wobble pair, and base stacking is preserved in the duplex, explaining the stability of the duplex. This structure elucidation provides information about the influence of a (T)(4) fragment on local helix geometries as well as on the nature of the T.T mismatch base pairing in a TTTT tract.