AbstractOligodeoxyribonucleotides containing 7‐deaza‐2′‐deoxy‐8‐methylguanosine (m8c7Gd;2b) were prepared. For this purpose, the phosphonate3aand the phosphoramidite3bwere synthesized and employed in solidphase oligodeoxyribonucleotide synthesis. The structures and the thermodynamic data of duplex formation of oligodeoxyribonucleotides containing2bwere investigated by temperature‐dependent CD and UV spectra and compared with those containing 7‐deaza‐2′‐deoxy‐7‐methylguanosine (m7c7Gd) or 7‐deaza‐2′‐deoxy‐guanosine (c7Gd;2a). In general, compound2breduces the duplex stability. In case of the sequence d(m8c7G‐C)4(18), the B → Z transition was facilitated by the incorporation of2b. Moreover, a single 7‐deaza‐8‐methylguanine residue present in an oligodeoxyribonucleotide tract of guanine residues destabilizes the dG quadruplex significantly. This destabilization is more pronounced than in the case