CD and NMR studies on heterochiral oligodeoxynucleotides (d/l-ODNs) forming quadruplex structures are reported. Heterochiral ODNs, based on sequence TGGGGT, are able to form stable either right- or left-handed quadruplexes depending on d/l ratio and residues position. Results suggest that the 3'-end and the core of the G-run are more important than the 5'-end in determining the quadruplex handness. Particularly, oligonucleotide T(D)G(D)G(L)G(L)G(D)T(D) (L34) at low temperatures forms a well-defined left-handed quadruplex, notwithstanding it is mostly composed by natural d residues. This structure is characterized by three all-anti G-tetrads and one all-syn G-tetrad.