Oligonucleotides incorporating the regioisomeric 4-nitroindazole N1- and N2-(2′-deoxy-β-D-ribofuranosides) 7 and 8 were synthesized and their base-pairing properties investigated. For solid-phasesynthesis, the phosphoramidites 11 and 12 were prepared. Oligonucleotides containing the buildingblock 7 or 8 show ambiguous base pairing. Duplexes have similar Tm values when the modified bases are positioned