In this paper we report the synthesis and the structural characterization of two modified oligodeoxyribonucleotides (ODNs), namely d(A 8Me GGGT) and d(TA 8Me GGGT), where A 8Me represents a 8-methyl-2′-deoxyadenosine. Both ODNs have been studied by 1 H NMR, CD spectroscopy and molecular modelling and shown to form fourfolds symmetric G-quadruplex structures, with all strands parallel and equivalent to each other. The complexes are characterized by thermal stabilities comparable to that of their natural counterparts. NOE patterns involving 8-methyl group in A 8Me residues allowed us to define the main structural features at the 5′-end of the complexes. Particularly, inter- and intrastrand NOEs show a syn-orientation and a symmetrical arrangement of A 8Me bases stacking on the adjacent G-tetrad.