Analysis of the Hoogsteen base pairs of guanine and 8-aminoguanine with cytosine in the gase phase shows a strong stabilization of the 8-aminoguanine.cytosine base pair due to the formation of an extra H-bond. Melting profiles of oligonucleotides carrying 8-amino-2'-deoxyguanosine confirm the triple helix stabilization properties of 8-aminoguanine.