The preparation of oligonucleotides containing 8-bromo-2′-deoxyguanosine is described. Substitution of G by 8-bromoguanine on an alternating CG decamer stabilizes the Z-form in such a way that the B-form was not observed. Melting temperatures showed that duplexes in which 8-bromo-2′-deoxyguanosine paired with natural bases were much less stable.