The oligodeoxyribonucleotides (ODN) containing C-2 and/or C-5 substituted arabinofuranosyluracils were synthesized by a post-synthetic modification method. The duplex stability of the ODN bearing isocytosine derivative with DNA was lower than that of the normal ODN/DNA duplex but the ODN bearing C5-substituted uracil derivative was similar to the normal ODN/DNA duplex.