Oligonucleotides incorporating 8-aza-7-dazapurines (pyrazolo[3,4-d]pyrimidines) were synthesized. The corresponding nucleosides were prepared and were converted into phosphoramidites. The oligonucleotide duplex stability was studied and was compared to that of the parent compounds containing the canonical purine nucleosides. The presence of 7-halogeno or 7-alkynyl substituents increases the duplex stability significantly.