Theoretical calculations on double and triple helices containing 8-amino-2'-deoxyadenosine were made to analyze the possible differences in base pairing properties between 8-aminoadenine and adenine. These calculations indicate a strong preferential stabilization of the triplex over the duplex when adenine is replaced by 8-aminoadenine. In addition, a protected phosphoramidite derivative of 8-amino-2'-deoxyadenosine was prepared for the introduction of 8-aminoadenine into synthetic oligonucleotides using the phosphite-triester approach. DNA triple helical structures are normally observed at acidic pH. However, when oligonucleotides carrying 8-aminoadenine are used, very stable triple helical structures can be observed even at neutral pH. Biological applications of triple helices could benefit from the use of 8-aminoadenine derivatives.